Bought some panels to go behind the seats on the sides to cover up some wires and ugly sheet metal, had to trim,made out of cardboard, but now how is best or original way of attaching them? I considered two sided tape or velcro, any ideas?

on a 70, the sails had two large bolts that also held the shoulder belt, as well two screws that held a clip which then held the belt from flapping. The cardboard was covered with the same material as the headliner.