i recently started putting together a super tiny NAS, with rust disks for storage. but the compact case i bought (a Silverstone Sugo SG13) only has real, usable space for one 3.5" HDD. i found a mounting bracket/plate (original, linked) that would allow me to an add an extra, second hard drive, but it had a few problems for my application and i saw some room for improvements.

my Western Digital Helium drives have “alternate bottom hole” positions (https://support.wdc.com/images/kb/2579-771970-A03.pdf; section 4.1.1) that the original mounting plate did not accommodate – and the holes it does have were ever so slightly too small in diameter for the HDD mounting screws i purchased from my local hardware store.

i bored out those holes a bit, and i made the plate section taller in each direction by a few millimeters, to extend the distance between the holes and the edges. i also thickened the plate by a couple millimeters for improved rigidity.

my hard drive fits well, and the whole assembly feels reasonably solid… however, this is still a ~50g piece of plastic supporting a hard drive that's about fourteen times its own weight. in a standard-layout pc case, with PCI cards mounted horizontally, this will likely not support a hard drive safely. but for a case with vertically-oriented PCI slots, this works okay.

i also created a second version, with “legs” extending to the bottom floor of my case. this takes the weight off the PCI mounting bracket/screw, and distributes it more safely. you could easily modify this design to fit your own compact mATX or ITX case.

i've printed v1 (no legs) in Hatchbox PLA at 30% infill (cubic).

i've printed v2 (with legs) in Polymaker PolyTerra PLA at 25% infill (cubic).

i'm happy with the results of both.  make sure you use supports, because of the PCI bracket hole and the overhanging flange.