For Revision 2.0+ Raspberry Pis with mounting holes. I made this so people would have a reference. I measured these with calipers, and not with official alignments, but as the pictures show they line up. After I measured, I found the mounting hole locations on the raspberrypi.org site ...heh. I wasn't able to find them prior, I swear!

raspberrypi.org/archives/tag/mounting-holes

I measured my friend's Pi, (as I lacked one with mounting holes) and found that the length of the Pi was 84.8mm, and not the 85.6mm referenced...everywhere. Ditto with width. Mileage may vary. Also don't over tighten the screws. :)

I used 3mm acrylic.
Blue is for inside cuts, cut those first. I pasted in the Ethernet standoff thingy from Adafruit's Pi Case.
Cut the red next to cut out a couple of acrylic standoffs.
Cut green last to cut out plate.

Feel free to use the plate as a safe way to place the Pi on a table, or use the two mounting hole alignments for reference elsewhere. (It's what I plan to do)

I placed the washers and Ethernet port standoff in one file to save on material, feel free to move them.