After trying some of the other options I'd seen online I wasn't particularly happy with the ergonomics of any of them and I definitely didn't like using bolts or dowels to make a simple hinge.

So I sat down and taught myself enough Fusion 360 to model this from scratch!

It prints nicely at 0.16-0.20 layer height from a .40 nozzle with no supports. All of the overhang angles are steeper than 45 degrees and the the hinge has at least .3mm of clearance so even with a relatively sloppy printer you should get some decently smooth motion.

The closure uses two “2mmx6mm” magnets stacked in each opening; make sure to orient the magnets in the correct direction, then put a drop of glue in the hole and push them in. They really pull the case shut with a satisfying SNAP! and keep it securely closed so your retainer doesn't end up in the bottom of your bag.

Per some feedback from my daughter I added some vent/drain holes in the bottom to keep the inside from getting funky :)

I'm really happy with how this thing turned out and it was a great learning experience.

This is Version 2 of my design, with smaller hinges that have steeper angles which should help people who had issues with the overhangs in the original.