I set out to design a case for my Raspberry Pi 2 (which, by the way, is about the sweetest Linux desktop computer $35 can buy!) Rather than design the case directly, I decided to first model the Pi itself, which I could then design my case around.
Everything was done in Google SketchUp within the space of about 1.5 hours. For measurements, I used a Neiko stainless steel digital caliper (highly recommended, and like the Pi, I got quite a good deal on it!)
Here's the case I designed:
Raspberry Pi 2 case (minimalistic)