I don't get to this side of the community often since I normally build planes, so I thought I'd drop in and say hi!
There is a guy here in Nashville, TN who recently made a couple of display cases for my planes. When I went to pick them up had asked me if I need a larger case because he had one that he had made for someone who never came and got it. It is approximately 24" x 36" x 6" that he said he'd let go for $25 (plus shipping, presumably). Note I said approximately on the size! If the size is critical drop him a note and ask him to verify it. I saw it and it would be real nice for a diorama. His web site is at http://www.bondplastics.com/ if anyone is interested.
He does real good work, but the cover isn't a single molded piece. It's a flat piece that is rolled to form two sides, and then pieces are fitted into the other two sides. I'm real pleased with the ones he did for my planes. They are 14 x 14 x 6 (dimensions I specified) and cost $20 each.