"The obvious solution is to use your window as a place to rest your lens on, like a hobo tripod. The thing is that the window edge is not the most comfortable object in the world to rest a lens on.
One thing that you can do is keep a bean bag in the car. They are small, provide excellent mount and you can even make them yourself. Now you can roll down the window, place a bean bag on the car and TADA!! An instant tripod"