On the securing bikes on a rack---I have a 10ft hawser and I link that through the bike frames and wheels and then the open end is passed through the Boot and secured to an immovable object within the car.
It can be seen in the pic and is the grey item on the left of the securing strap that runs into the back of the car.
I have used this basic Carrier for years and never marked a car in any way----Except that the fixings on the straps have strips cut from an inner tube to act as a buffer. The wheels and bars have to be located firmly so they don't swing and hit the bodywork and this is done with the luggage elastics that hold the bike onto the carrier.