Surprised this thing didn't already exist! I had to hastily hack together my own way of embedding a Raspberry PI 2 or 3 in a regular PC tower :)

This borrows some OpenSCAD code from https://github.com/Petraea/PCI-Bracket

You will need four self-tapping 2mm x 6mm screws or similar to fasten the Pi. I am planning to power my Pi with a Molex 5v to microusb cable, but I suspect any spare USB3 port should provide enough power.

Unfortunately it is a tiny bit wider than a single slot so it might block the card next to it.