Instead of foam tape that degrades I use Goop. Its one of my favorite adhesives. It forms a semi-permanant bond (eg it will stay forever and gives a strong bond of anything to anything but if you ever need to you can remove it by peeling it off). I used goop on the back of my license plate to protect the mounting area from the sharp metal of the plate. Just put a thin bead on the back of the plate and let it cure up then mount the plate. I use the Marine grade but I don't think there is any real difference from what I can tell in the various "versions" of Goop. Shoe Goo works good too but I can usually get Goop cheaper....
