I built my own Camera Mount and it was incredibly simple and incredibly stiff, there was no shake at all.

All I did was take a 1'' aluminum bar, cut it to about 1.5' drill two holes in it big enough and spaced so that the headrest would go through it, then I drilled one hole for a screw that went into the bottom of the camera. So basically a bar with 3 holes, very simple, and works very well, I can get some pictures tomorrow if you wanna see.