I've used truck cap/camper foam mounting seal in the past and it has worked well over long periods of time. Real inexpensive too so it can be replaced often since any footpads, etc will collect dust, dirt and other grime from the road requiring replacement eventually.
It's easy to cut/trim to whatever you need and it'll stick well to whichever surface you need.
Another product I've used is the Scotch 2228 Rubber Mastic Tape (Home Depot has it...) and that stuff is incredible. It's not a foam, but a rubber, so it may collect more dust, etc, but it will conform to anything, stick forever, and has enough give to the material to use as a bump stop, etc.
No matter what you use, be sure to check and clean the surface often.