If you are german, you probabely know Altenburger Spielkarten. What do they come in? Right, a nice plastic case, nothing fancy, but easy to cut and just the right size for a raspi, as it turns out!
So if you are looking for a cheap, fast and easy to make case, just go to the next toystore, and get yours (just make sure to get the poker deck, a skat deck is too thin)! Photos of my build coming up.