I doubt you'll get the angle from the rear window. I'm going to put a camera on the spoiler looking down and have the camera in wide mode. In this position you can use a tether to literally tie down the mount to the spoiler. Even if the sucky mount lets go, the camera can't move and do any damage.
Will also experiment with an extension arm to get the camera a bit further out.
Was going to do it this evening but it's just started raining....back to the snooker I guess!