: . . . If you really wanted to go "offshore" you could make a hard ply
: cover to cap your hatch "cockpit" and reinforce the shower cap.
: But most of the time I am betting it wouldn't be necessary. . .

That is the right theory. build up a lip of a caoming, not for a cockpit, but for a hatch, and then cap it with a piece of plywood that has some squishy material inside it to seal things.

-- and then, after you have this designed, recess the whole thing about 1/2 inch into the kayak by putting in a ring of plywood and appropriate spacing materials just under the hatch opening and building on that. Then, when you are done, assuming you measured things right and allowed for the compression of whatever sealing material you used, the top of your hatch will be flush with the top of your kayak deck.

how you hold this hatch on will be the next challenge.

A hatch which sits above the deck can be held down by nylon straps. They don't apply a lot of downward pressure, but they do apply some. But when the hatch is flush, those straps apply almost no downward force on the hatch. So the quest for hatch latches comes into play. One current appraoch is to use bungie cords under the deck that attach to the underside of the hatch. These are also called "Leidy Lids" after the builder who developed and popularized the approach. There are other methods, and the search for new ideas continues. Maybe you'll dream up something interesting and let us know about it.

PGJ