This is a modification of Adafruit's Raspberry Pi case. Please don't let this cut into her sales, so if you decide to make this please buy some stuff from her and throw your money into the interwebs. I'm not sure if this works using the slightly thinner plex, but I have one on the way to try. If you decide to give it a go, godspeed and good luck.
UPDATE: I tried it, and it works pretty well. It rattles a bit, but I expect the same would be true of Adafruit's case as well. I think it turned out well, and I'd feel comfortable recommending it now.