Steve Perry wrote:
Honestly, I usually don't use anything. I just get the window to the right height, shut off the motor (that's the critical part) and shoot. Been doing it that way for years. However, if you want to put something there, I'd try a short piece of pipe insulation. Cheap, quick, and easy.
Here's a few shot with the lens right on the window glass - the geese were actually published in Outdoor Photographer Magazine (Nov 2011, Last Frame).
Download for the better versions.
I read a story several years ago about a guy who made a heavy, thick, three-compartment leather beanbag, with a 2" x 10" platform in the top center compartment, and a 3/8" tripod head stud coming through the top of the leather. He mounted a Bogen fluid head on it, and made the side compartments heavy by filling them with BB shot. He would drape this rig over the open car door, and use it as if it were a pro video tripod.
It was tailor-made for his car door, and it worked perfectly... until animals wanted food! Fortunately, a quick lift into the car, power windows, and a clear getaway path solved that problem.