If you have an old ATX computer case and power supply lying around, why not use it to expand the storage of your main PC? Turn it into a DAS! (Direct-Attached Storage)

This design mounts 4 disk racks in the space that would normally be occupied by the motherboard, with each rack supporting 4x 3.5" disks, for a total of 16 disks. 

SFF-8088 cables (one per 4 disks) connect a SAS controller card inside your PC to the DAS, which then fans out to 16 ordinary SATA connectors to connect to the disks.

The printed parts are designed to fit on a Prusa i3 MK3's printbed (the largest is 244×153mm). Only ABS has been tested (specifically eSun ABS+ Black and Fire Engine Red). PLA will not work because it'll bend like a wet noodle once the system gets up to temperature. PETG might work, but the clip geometry was optimised for ABS.

Bill of materials

• ATX computer case (note, not MicroATX, must be standard ATX size). I'm using the be quiet! PURE BASE 500 because it's cheap and came with two 140mm fans
• ATX power supply (with SATA power splitters 🅓 if needed to get enough connectors for your drives)
• A SAS controller card for your PC with external SFF-8088 SAS connectors. e.g. LSI controller cards with model names ending in 8e or 16e (supporting 8 or 16 external drives respectively). ZFS users will want a controller flashable to use “IT mode" so that they don't have to suffer disks hidden behind hardware RAID.
• 🅐 2mm / 5/64" hex driver with ball end
• 🅑 #6-32 ¼" flat-head (CSK) screws with hex drive, 86 pieces (McMaster)
• 🅒 #6-32 hex nut, 12 pieces (McMaster)
• 🅔 SFF-8088 to SFF-8087 adapter brackets to convert the external SAS cable to internal  SAS inside the DAS, one bracket per 8 disks, 3 brackets maximum (AliExpress)
• 🅕 SFF-8087 to 4x SATA breakout cable for connecting the disks to the adapter brackets, one cable per 4 disks (AliExpress)
• 🅖 Fan speed controller (e.g. this controller for 3-pin fans from AliExpress)
• 🅗 SFF-8088 external SAS cables to connect your PC to the DAS, one cable per 4 disks (AliExpress)
• (optional) 140mm or 120mm case fans to mount to the drive racks, up to 2 pieces
• (optional) cable-ties 3.5mm wide or skinnier

Expected build cost, not including the case or power supply (since these will vary), but including $36 for an LSI SAS controller card from eBay, all other non-optional parts, shipping from AliExpress, and ABS+ filament, is approximately:

• 4 disks - 103 USD
• 8 disks - 130 USD
• 12 disks - 177 USD
• 16 disks - 204 USD (includes $17.10 for 690g filament)

Checking your case for compatibility

Not all ATX cases will be compatible. The printed baseplate is the size of a standard ATX motherboard (12 × 9.6 in, 305 × 244 mm), but drives in the lower racks will overhang the lower edge of the board by 2mm:

The racks are very tall, quite unlike a regular motherboard, so there need to be no overhanging computer components in any quadrant you want to install a rack into. Rear case fans will certainly need to be removed.

The tip of the drive rails stand 182mm tall from the top of the motherboard standoffs, so your case ought to advertise a “CPU cooler height” compatibility of at least this much.

Printing

Print in ABS, at 0.2mm layer height, 3 perimeters, infill 15% rectilinear. I used PrusaSlicer's new Arachne perimeter generator and turned off elephant foot compensation.

A note for ABS printing, since ABS shrinks after printing you'll need compensation enabled to get everything to come out the right dimension. In PrusaSlicer I had to set the X and Y scale on the objects (but not Z) to 100.76%. The equivalent for SuperSlicer is setting the Filament Shrinkage setting to 99.2%.

The baseplate is split into two parts, a Bottom and Top half, and you ought to print both, plus two copies of the Joiner Plate.

Each rack is composed of a pair of Left and Right Wall pieces, so you need these for each rack you plan to install. 

For the bottom-left rack you need to print the “Rack wall - Left with PCI cutouts” version for its left side, for the rest of the rack walls you can use the left and right “- for airflow” variants.

You need to print two drive rails per drive. These need to be printed with 5 perimeters (to make them fully solid, with filament oriented lengthwise) and I also needed to enable the brim to prevent them from warping off the buildplate during printing.

Assembly

If you will be installing 140mm or 120mm fans to the rack, first install these to the outside of the right-side rack walls. The fan mounting holes are marked here in blue, the outer square is for 140mm fans and the inner square for 120mm. You can use the fan's included screws. Screwheads must lie below or flush with the printed surface for clearance with disks.

Three motherboard mounting holes on the right side of the baseplates will be covered up by the rack walls once they're installed, so you need to put screws into those holes now (highlighted blue below) before going further. Add a nut to the other side to temporarily hold the screws in place while you're installing the rack walls:

Now you can install the rack walls by pushing their clips into the slots. Note that the left and right walls are not interchangeable, don't plug in the wrong ones! Each rack has a pair of left and right walls. You can tell if you've chosen the wrong wall because its edge won't be flush with the edge of the baseplate.

Correctly installed, rail slots face inwards, blue edges are flush with plate edge:

Incorrectly installed, the rack edges don't align with the edge of the plate, and overhang the far side of the plate:

Now you can remove the nuts that were temporarily holding the 3 trapped screws in place.

Join the top and bottom baseplates by using the two joiner plates, with screws entering from the top and nuts on the bottom. The screwholes in the baseplate are covered up by a sacrificial bridge for printability, but you should be able to easily pierce this by pushing the screw through (as a bonus it holds the screw in place while you're working).

Note that the notch in the joiner plate needs to be aligned to allow access to this motherboard screw hole later on (red arrow):

It should look like this now:

Underside detail:

Installation into the case

Add the internal SATA cables to your PCI brackets now, and slip the cables and the rear of the cards through the PCI apertures in the bottom-left rack wall.

These three screwholes in the bottom left corner might be covered up by a lip on the edge of your case during and after installation:

So slip screws into those holes now before sliding the plate into place on the motherboard standoffs and the PCI brackets into their slots.

Once installed, you can tighten these hidden screws (red) by slipping your ball-end hex driver through the apertures provided in the Left wall. These apertures afford a 15-degree angle of attack on the screwhead:

If you get tired of playing keyhole surgeon, you can temporarily remove some PCI slot covers to give you better access to the screws (the rack wall will bend out of the way for you to give access to these).

You can now screw down the rest of the motherboard screws and your PCI brackets if you didn't already. When you're done it'll look like so:

Adding drives

Screw a rail to each side of your disks like so, with the latch/handle at the connector end. Don't overtighten these screws, since it could split the rails (they don't need to be very tight anyway):

Drives install into the racks circuitboard-side up. When you're sliding the drive in, squeeze the sides of the rack together to ensure that both rails slip into their retention pockets inside the walls. The rails pull the rack walls together to make them rigid and square. You'll notice the rack becomes solid as soon as one disk is installed into it.

Push the disk in firmly until both clips engage in the rack wall (you may need to push the clips outwards to help them out).

Removal is the opposite of installation, squeeze both handles inwards so that their clips release, then pull the drive out.

For the disks next to PCI cards, verify that their PCB is not shorting out against the card. Here the metal tab on the SAS cable comes fairly close to those green blobs on the hub of the disk, so I added electrical tape to the disk to ensure there wouldn't be any trouble in the future:

Cabling

Cable channels and cable-tie holes have been provided in the top of the rack walls so that you can route cables between racks:

However my cabling is a mess because some of my rack walls predate the addition of that feature, lol. Wiring in progress:

I stuck the fan speed controller to the back of the motherboard tray using its included adhesive foam:

Since we don't have a motherboard to operate the case's power button, we need to instead tell the power supply to turn on by shorting pin 4 and 5 on the motherboard power connector (green wire and black wire). This causes the power to be on all the time (you can turn it off using the switch on the power supply instead).

The traditional way to do this is to stick a paperclip into those two connector pins. Since I won't need this power supply in a regular computer again, I instead snipped those two wires from the connector, stripped them, soldered them together, and covered the join with heatshrink tubing:

Now all that's left is to plug your SAS controller card into your PC, and connect your PC and the DAS together using the SFF-8088 external SAS cables.