Most of the commercial solutions for resting the lens on the car window are bulky and perhaps heavy and thus not very good for travel. I tried a bag of beans but had difficulty keeping it on the window - need three hands.
Then it hit me. I bought a pool noodle, cut it down to about 18" and slit it. It fits over the window, stays put and has excellent feel for resting the lens. It weighs about nothing and I have at least two more pieces left to make from the original noodle. I chose blue. Your preference may differ.
Our drive-by bird photos just got a little better.