: Hi Brian,
: My bigger concern would be with expansion and contraction. Of course a deck
: plate in the bulkhead would take care of this also but would be hard to reach. : My thinking is the amount of air trapped would be small and the surrounding
: construction so strong this wouldn't be a problem. Is this a flawed idea?

I don't like the idea of any completely sealed compartment. The forces created by the thermal expansion of air can be very high. Too me, it doesn't seem worth the risk. If you're going to seal the compartment, fill it with styrofoam or something to reduce the amount of air trapped within.

: Then again aren't there valves that would
: allow for air pressure adjustment without letting in non pressurized water?

None that I am aware of. If you have any Gore-Tex fabric kicking around, you could make waterproof/breathable "valves" with it. It may sound a bit goofy, but it should work.