Er - I didn't think a junk mast had a track! To be serious, one of the 'old-fashioned' sealers, if any still exist that usually purported to stick but didn't would work. I might be inclined to fit a spongy rubber gasket, myself. With such a big surface and such a weight on top, it should work. You could even contact-glue it to the deck so that you wouldn't get any leak at that level anyway.
I have heard, but never tried it, that a length of thin piano wire (or guitar string?), each end wrapped around a stick, makes an effective saw for cutting through very thin, flexible glue joints.