A 5.25" inch bay to 3.5" HDD adapter, from 4 bays to 5 disks.
Print two copies of the frame size you want, screw them to your case (a couple M3 screws) and slide in your hard drives. Drives are held snugly in place by friction, with space for airflow.
I've included versions for 1x, 2x and 3x bays, but the model was designed for 4x.
If you're feeling extremely impatient, there's a half-depth lite version as well, which I haven't physically tested.
Not much infill required, I went with 10% and it's fine. Print with three walls if you're worried.
Remember to print hot, so the layers bond well!