aeroskunk

sounds good.. to keep the light leaks off every other connecting piece i just used some wood glue around the edges before i screwed the backing/frame together.. so it guaranteed everything was real tight, and tapped the screws about every 4" .. but duct tape will probably be the easiest repair in your scenario.. i spose.. i'd even look into some lightproof caulking.. just a strip down each corner on the inside of the box should work.

ButterflyDreams

Glad to see you're putting all that study into action.

For the corners get a tube of bathroom type caulk. It'll handle the moisture and temp perfectly. Sounds like you should be able to get inside and hit all the corners pretty easy. Then wet your finger and smooth it out. Let it dry and, bam, no more light leaks.

The duct tape will work, but it has a tendency to pull loose when around moisture. If you do use it, don't get the super cheap stuff. Pay a few more $$ to get the kind that's super sticky.

HD or other hardware outfits will often have "in-line" style vent fans available in 4" and 6" setups. They are designed to fit inside a round duct to assist the heat/AC for those rooms that are farther from the unit. I helped a buddy set one up. Use the spray foam as a shock absorber and the sound issue is nil.

Smiles,
BD

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