Re: 3M 5200 as a gasket?
5200 can work as a gasket. When I install permanent deck hardware I'll typically bed with it but not crank down the fasteners all the way. Just enough to give the goop a slight smoosh. Than about 5 days later, when its cured, I torque the fasteners to where they want to be be. Provides a better seal, in my view, used this way.
For your purposes, you would need to line the lid that makes contact with the flange with plastic ( or some other release agent--I don't know, not having done exactly what you propose before). Apply the 5200, close the lid without dogging it down, and then wait for it to cure. When cured, remove the release film. If it works great; if not you can remove it with heat and a scraper, which shouldnt be that hard on metal tanks.
There are two kinds of boaters: those who have run aground, and those who lie about it.