I built my case custom. I used my harddrive box as a template, because it was just large enough to fix my flex ATX mobo. I've found this ended up being a good size, it allows just enough room to not overhead, yet since its small and wooden, it heats up quickly for the HDD in the winter. I'd suggest fashioning something yourself.
My only regrets were that I didnt design my trunk layout to accomidate it....so its stilljust sitting out in the middle of nowhere, about the side of a shoebox.