It can be mounted on most anything, so long as the surface is stable and won't bend or anything. Of course, without the proper enclosure, you may not have proper shielding, which can create problems, but physically, it can be mounted on most anything. Also, if you mount it on metal, make sure it doesn't touch the metal. That's A Bad Thing.
If you are looking for the smallest case you can get, I used a Klein tool box that is only slightly bigger than the motherboard. It is tall enough to wedge in an IDE harddrive above the mobo with about 1/18" to spare. It's pretty easy to work with and will give you the shielding you'll need for a car.
(Klein Cat# 54439)
i built my mp3 player into a truck subwoofer box. there was plenty of room, and it fits behind the seat in my truck conveniently and without looking out of place. the only drawback i can say is that the case is too heavy making it a pain to carry around
K6-2 450 SBC, miniature case, dc power supply, 5gb laptop hdd