If you want it removable go to the weatherstrip section of your local big box and get CLOSED CELL neoprene gasket 1/4 thick by 1/2"
Edit: Home Depot has it with the floor registers for some reason.
If you don't care you can just RTV (silicone) it in place.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.