There are a lot of diy cases on the net. Probably the most notable one is the cardboard case. You can easily find them on the net by looking for RPi printable cases. Be sure and tell the printing program not to resize image for printing.
This next two were not real impressive and not so easily put together. Kind of flimsy?
This next one seemed a bit complicated.
Then there are the .dxf files with Librecad so you can use them a laser cutter or cnc router.