Upload & Sell: On
I have tried a variety of vehicle shooting solutions... everything from the Groofwin pod to the Kirk mount to the Eckla Eagle device. And I've also tried an Apex bean bag device which enables you to use a ball head or Wimberley on a bean bag assuming your vehicle has enough room inside (mine did not). Ultimately though, I decided on a bean bag designed by Art Morris called a BLUBB. I find that for shooting with both a 500 and a 400 DO, it's been my best solution. But I've also used a monopod, as outlined by a previous post, and that has worked adequately. It's really all what works best for you.
I will say that if all you're shooting with is a 100-400, something along the lines of a BLUBB should work for you just fine as it has enough mass to stay on the door/window, as well as cradle the lens securely