Anyone planning to put a fiberglass cap on their truck needs to checkout "truckcapseals.com" before allowing the dealer you purchase the cap from making the installation. Most dealers use some little ol' thin bubble foam material with adhesive backing as the gasketing between the cap and the truck sides. It's cheap and it does not really do much as a barrier between the cap and sides. Clamping the cap to the sides with this material in between the two does not stop the cap from shifting position over time. The cap seals from Missouri Polymers (truckcapseals.com) are a great product. Used it on the last three trucks I've owned with no complaints. Telephone your order in and it's less costly than the internet pricing. Shipping is prompt. With minimal effort you can make that box waterproof and dust proof.

Wow! That's the real-deal seal. http://www.dieselplace.com/forum/smileys/Approve.gif

I was thinking that I have seen this sort of thing a Lowes and Home Depot. I think it was for the bottom of garage doors.