Funky Foam 12" x 18" Black EVA Foam Sheet - 2mm | Shop Hobby Lobby
Sometimes it's listed as "foam", but it's neoprene or like a neoprene. 2mm is plenty. Just seal it in with the spray bed liner. Between the foam/neoprene barrier and the bed liner seal you won't have any issues with rocks starring your exterior. No bleed through. I don't have pics to post but this is a one beer project.