Weatherstripping from lowes/hd. Get the 3/4" wide 3/8" thick. Stuff works wonders for sealing up things like door panels and speaker mounts, stopping pieces from slapping together and causing panel rattle, etc. It costs like $4 for a 10' roll. I have at least 4 rolls of that in my pathfinder. A roll on my back hatch that goes around my rear windshield (cut down on the flex by at least half and it no longer slaps and makes any noise), a roll per front door and a roll on my roof to stop my headliner from slapping the roof braces.