I've thought about making something along the lines of this.
Long strip of aluminium about 3" deep bent up at the ends with a bit of rubber glued in the bend to protect your rear windows & maybe a strip of pine or something screwed/glued underneath to strengthen it, then 2 or 3 different mount holes for the camera position...........you can move it back & forward till it's in the right position before clamping it in place in your back windows.
Crude but cheap & excuse the piss poor drawing effort but you should get the idea.
