Found a couple of photos which might be of interest:
The aluminium L-shaped part gives stiffness to the plastic/perspex plate. The horizontal part of it could be narrower. It is attached with pop rivets if memory serves me well.
Between the plastic plate and the boat, there is a piece of yellow foam which was cut from a cheap camping matress, and it is kept in place with small bolts and big washers cut out of something plastic and transparent (so you can’t see it well in the photo 3).
The clever idea is the pair of bolts on which the flap is hinged. They are screwed into the transom (holes in the transom were threaded with the No1 bit – sorry I don’t know the proper term in English), but also have nuts and washers on inner side of boat for stiffness. Holes in the flap through which the bolt goes are somewhat large and sanded (wrong word again probably sorry) so that the flap is quite loose if the bungee cord on the inside of the boat is undone. But still, these holes are smaller then the head of the bolt so that the flap can’t drop out.
Hopefully this makes sense…
The whole setup is somewhat unrefined with regards to size and it’s weight, but this was meant as a first experiment with a new idea (flaps hinged on bolts) which would be refined if found to work.
I’ve slept in the boat so I know that this system doesn’t leak at all, and is easily released in emergency. So it does work, but just as Swiebertje said, even the amount of water that gets caught inside in a partial capsize, will make the boat so heavy that water will run into it just as easily as we’d hope it would run out. One really needs to start sailing the boat again, and then the water will go away through bailers, and the difference in speed of draining is not noticable in my experience with flaps loose, or closed.
So, really the best thing with a WWorld is to close those things forever and have one complication less in the boat.