I've used an aluminum case that measures 12"x10"x3" and just cut the necessary holes for cables, fans and cooling with my dremel. (gotta love dremels) This way I could put the unit where my spare tire goes in the trunk under the "floor".
The case I just ordered from an electronics store and the actually manufacture is called Bud Industries. They make all sorts of boxes.
I will have some pictures up somewhere when I get my digital camera back.