question about subwoofer install
 

I got this fiberglass enclosure for a 8" sub that goes in the passenger footwell, and I would like to know how to go about mounting the sub into the enclosure so that there are no air gaps.

What do I do besides drilling the screws of the sub into the enclosure?

Most of the subs I've installed had cork or rubber gaskets on the back to get a good seal. You could always make something like that. In most applications though I've just used a bead of silicone caulk. The caulk will seal the sub tight against the enclosure, it will help to hold the sub in tight, and the silicone is easy to remove (just peel off) later if you want to do something different.