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My 21' was a bare hull when I got it. I cut up some dock foam like you did and made a tank cover out of fiberglassed plywood and put an inspection port in it. The foam degraded like yours and I was always having to pump out the cavity. A few years ago I drilled a small drain tube to the sump and installed a small automatic pump in the sump, the kind that comes on every 2-3 minutes. So now there is only a small amount of water at the bottom of the tank well and sump. I am going to have to make my tank cover over again ( 18 years old now ) and am thinking of filling it with the two part foam mixture which is probably expensive. That well probably is at least 50 gallons. Maybee there is some other dock foam that will hold up better or some other cheap material We can use. Anybody got any ideas? Good luck Tom.