for interior seams, I've always used '25 year' silicone caulk . . .
for the woofer opening, you definitely want something non-permanent - weatherstripping works, but you could just use what they used back in the day - plumbers putty :yes:
just make sure to clean both mating surfaces, and if you really want to go crazy, use T-nuts to mount the woofer, instead of the original wood screws . .