If the road is VERY bumpy, as in off-road conditions, (almost) any kind of rig will get some bumps.
It the conditions aren't that bad, the cheapest way to do this is to hold your arms attached close to your chest and hold the camera with your left hand beneath it and your right hand on the right side. If you've preset your focus and have pre-framed your shot, ideally NOT on maximum telephoto but as wide as possible (but not so wide that you'll be including the inside of car in the shot), and you concentrate and relax, without stiffening your arms, this grip will allow you to act as a natural shock absorber.
Usually, this method has worked just fine for me. Some bumps are "natural"; by this I mean that if it is clearly visible that there are bumps in the road, the occasional bump won't disturb the viewer because they will see that you are travelling on bumpy terrain.
As a matter of fact, all of those mounts – from the indicated web site – are quite rigid and would work well only on a normal, smooth, highway.
I'm quite confident that if you're not using a large camera for this shot – I normally use my small (second camera) in these cases – the indications I have given you should allow you to manage just fine, unless (as I pointed out above) you're travelling on really bumpy terrain.
Edited Mon 7 Jul 2008
by Wolfgang Achtner