I'd like to do STRETCHED verison of this roof-top cargo-box/dingy in Stitch&Glue.

https://boatboxint.com/collections/boat-box

I'm thinking just under 10ft so I can use full length sections from 10ft marine plywood, plus it about max roof box practical length and min practical boat length for anything but strictly ship to shore dingy.
But I've got an additional tricky feature I'd like. The bottom tray with raised sides is what makes it easy to load as it keeps stuff sorta in place, so that is a "keeper" feature. But I'd like to be able to flip the tray then set it in the boat hull to create a double hulled boat with SOT kayak type buoyancy (without drain holes) as an option. Question is: What sort of seal around the rims of these two halves?
As Cargo Box it needs to be rain proof and more or less wind driven rain proof for driving in rain, but doesn't need to be submerged in water proof. Really doesn't need to be extended submerge in water proof when acting as double hull boat. I guess a manual installation of some gasket or other such operation when transforming into boat would be OK. Also like to have it usable as single hulled boat probably with benches that would become structural when installed.
Might end up going with single hull function only and single function bottom tray function only.
As far as hinges I'm thinking of using total of six standard door type butt-hinges, 3 on each side on a straight section of gunnel. Idea will be that the hatch could open left or right by pulling pins and the center hinge on each side could be used to lock the whole thing shut with standard pad-locks or long shank padlocks would serve as hinge pins. These hinges would need to be flare out away from the gunnel-seal and could except some gizmo in the their hinge-pin holes to mount oar-locks. Seems they might make paddling awkward so maybe just two hinges far fore and aft.