After acetone-smoothing the mounting surface, I apply the 3M super strong auto tape to the side that faced the bumper. On the outward side, I installed brass thread inserts (Amazon or McMaster) that essentially melt their way into slightly-too-small holes in the plastic. You heat them up with a soldering iron and slide them in. They have knurling on the sides intended to prevent them from spinning and, once melted in, they are as strong as you'll ever need for a license plate!