Depends on the hide as a lot of shelves aren't very stable.
I have a Kirk Window mount and regularly use it with a 500 from the car and it can be used as a ground pod too. I've never cracked a window (although I admit it is possible) as the mount passes the weight into the car door and not through the glass.
For hide work, a Manfrotto SuperClamp and the accessory they do to take a tripod head can work very well indeed but it needs a sturdy shelf.