A 120mm fan bracket for 3x 5.25 inch bays. It mounts with M3 screws just like an optical drive or other 5.25 inch bay accessories.
This is a slightly refined version of https://www.thingiverse.com/thing:2446552, with many dimensions tweaked to give a better fit.
Don't have a 3D printer and want to buy a print of this? Please check out my Etsy shop!
https://www.etsy.com/listing/828166590/3x525-inch-bays-to-120mm-fan-mount-for
The author hasn't provided the model origin yet.