Hey Spidey

Very nice work, love what you're doing.

But what you're doing raises some ideas that maybe others could use(you're too far ahead.

Seat Attachment and Adjustment

Foam is easy to sand all kinds of ways so What if you sort of extrusion sanded a t slot in the bottom of you seat in the meatier areas just beside the butt. Or v-shaped like a dovetail. Glass the slot so it's fairly robust and rill 3 holes along ea runner that fits in the channel slot. Fit the runners and place the seat in the yak. Mark the holes, remove and glass in a ss tnut at ea hole Then replace the assy and screw in the runners/seat. Now have a sliding seat. Add a bungee to the back of ea runner to the seat and a short line to a tiny v clamp( the kind you just jam in place) at the front of ea runner. (So if you're in trouble at leas the seat goes back and not in the way)

If the runners are more on the sides or facing ea other, prob wont even need tslots or dovetail slots, they would stillhold the seat inplace.

Hatch

Another hatch latch idea sorta based on that set of tube and fittings posts and drawings I made last yr & is like an upside down tube approach.

So make a flush hatch and lip as usually made here. For the sake of argument, say the hath was blt of 2 layers of strips around the perimeter so the flange lip is lower by that extra layer. (like the hatch is 1/2in thick at the perimeter and the flange is down by say 1/2 plus gasket say 5/8in). And say the flange is 1 1/2 in wide instead of the usual 1in or so. Put in silicone or poly gasket after below.

Take some 1 webbin an make 4 or 6 loop fittings and fit under the flange. Cut matching slots inthe hatch and recess by one strip thickness. In the top of ea loop fitting cut and seal a slot like a button hole. Maybe soak the webbing so it/s sorta stiff. Set the silicone gasket in the usual way making sure it surrounds the webbing. Then place the hatch and push in some recess shaped strips with tiny button like knobs that fit in the webbin slots.

Or for concealed use real tight loops and make clip like wood fittings that match the strips on top and grab the loop below.