I ran something like that from Kirk many years ago. It can definitely work and does allow you to mount a tripod head like a gimbal on it but to me it wasn’t worth the effort to set up, tear down, store while not in use and it was one more bulky thing I didn’t want to travel with.
As Eric posted a lot of folks use a bean bag but I’ve gone to Steve’s simple solution of just carrying a short piece of foam pipe insulation to pad the top of the open window as discussed in this thread:
https://bcgforums.com/threads/my-in-car-window-support-strategy.1054/