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.
I use a 10mm brim combined with the default support generation in slic3r. This print will not work without support since parts of the bracket starts in the air.
Carefully bend the bottom of the bracket outwards a bit, it makes it easier to find the guide slot in the PC case when installing. Make sure to remove all support material from the hole for the ethernet cable, it's a tight fit. Depending on your case, the two upper USB ports will probably be blocked after installing. Also take care not to apply too much force, the plastic bracket is quite flimsy due to only being 1.2mm thick (you can increase the thickness in openscad but any more wouldn't fit my tower)