Scottie

A friend built me a box I was thinking of putting something on the outside seams and nails to seal it better something like glue, do you know of something I could use that will work good?

Dukk

Nothing. Unless you have obvious gaps at joints, the enclosure is sealed well enough already. If you want to waste time feeling productive though, mix up a litre of fiberglass resin, pour it in the hole in the baffle, and tilt the box around so it covers all the seams and all the wood inside. It won't do anything, but you'll think it did.