I built one out of an old bike rack (the kind that is supposed to strap to your trunk.) I added some suction cups, straps and a tray to support the camera. It works great.
Dan Selakovich shows how to build one in his book:
check out the cinesaddle - i made a cheap reproduction out of some canvas and packing peanuts (I'd get something closer to the bean bag stuff next time). Their website shows some great examples of how you can use it (including as a car mount)