How about hanging a tilt sensor from a string. When the car pulls in the tilt sensor moves to horizontal on the roof of the car. I don't think scratching paint would be a factor. Maybe I'll try it with one of my surplus Zooz tilt sensors. They didn't work mounted on the door, maybe they'll work hanging in the air.
Of course you could run into it with say, your head, but again, light and shouldn't scratch the finish. 
edit: People hang tennis balls on string to gauge where to stop the car. Maybe incorporate a tilt sensor, ie, put it inside a sliced tennis ball.