I'm definitely planning to mask off the deck and little recess, but I think this is my plan. I'm too lazy and cheap to go cutting plywood discs just to weight it. If I had easy shop access right now and big enough scraps laying about, I might, but I'm not going to buy plywood for it. I don't think the 5200 takes a lot of pressure to create a bond anyhow, does it? I think as long as those surfaces are in decent contact, bedded in the goo while it cures, it's going to stick. A few bricks or a gallon jug of water ought to provide enough pressure.

I'll just also tape off the lid/rim once I put the lids into the rims so nothing can get in the threads. I don't much care about squeeze out inside the kayak. These rims have a vertical flange on the inside that underhangs the deck by a few mm, so nothing should get into the threads that way (but I am going to double check this vertical flange goes past the screwed-in lid), and i'll just lay some plastic sheet inside to catch any drips. It's not like the kayak is a showroom model, it's pretty beat to crap already. I just want the dang hatches to quit leaking so bad. : )

Thanks, you gave me good stuff to think about.