I used a closed cell foam gasket strip, I think the same flat rectangular section as used on part of the cockpit locker seals. Stuck seal on the case. So far so good....
I also used silicon spray lube on the wood cap piece, hoping that this will prevent damage to the seal if/when removed...so that it should become a maintainable, resealable opening if needed.
The capping piece seals well enough, yet in a heavy sea, water now comes in around my bl00dy compass (mounted on capping top, aft), which i didn't seal!