Summary

This thing allows you to mount a 3.5" drive + a raspberry pi in a 5.25" bay. The intention is to use an external cdrom enclosure, which comes with both 5 volt power and an USB-to-SATA converter.

Instructions

Print the plated parts, file and fix the edges to that everything fits together smoothly. You will need:

4 standard case/drive screws
4 for holding everything inside the bay (same screws as above will work, also the smaller cdrom screws, but screws made for plastic is probably better)
2 screws for holding the raspberry pi, the small cdrom screws may work, but are a tight fit, smaller screws would be better. If you don't have anything, you can scavenge something from a cheap plastic toy. :)

Drill all the screw holes to fit the screws you have and start assembling. To power the raspberry pi, you can either dismantle an micro-usb cable and hook it up to the 5 volts in the enclosure, or if you're lucky, and your enclosure has molex connectors, you can re-wire an old molex-to-molex+floppy cable to do the job, which is what I did.

Finally, you'll need an usb connection from the raspberry pi to the usb-to-sata converter. You can either use an external cable, or just solder some wires. USB is fairly forgiving with short cables.

Beware that the USB-to-SATA converter that comes with most/some/all cdrom enclosures do not support drives large than 2Tb.

Raspberry Pi File Server