I can remember when i was working for fiat and alfa years ago.I fetted a reverse sensor kit and all it was,was a strip that looked like a thin piece of tin foil that stuck to the back of the bumper.

I know some reverse senor kits come with brackets to mount the sensors underneath the bumpers on vehicles.Im sure the sensors have to be at a certain height.

Have a quick look on ebay and see if there is any on there.