I used some 4mm (approximate) foam rubber, glued in with 3M high strength spray glue (actually contact cement in a spray can). Then I coated that with black rubberized truck bed liner (again, out of a spray can). Two coats of that stuff and I don't hear any pebbles on the underside. I did this to add protection from the sticky MTs. Just take your time and mask off everything. You also have to be steady with a razor blade and cut out each piece...sort of like putting together a jig saw puzzle while lying on your back.
-Dean