Next photo shows the problem I found when trial mounting the rudder. The trim strips across the transom stuck out far enough to seriously reduce the rudder movement. I cut them back to provide enough clearance.
I fitted the "pot lid" hatches to the fore and aft bulkheads. Here is a photo of the aft one. It will work well and the neoprene sealing strip will keep it watertight. My planned turn button clips to hold the hatch closed will work but not as well as I had hoped. I am rethinking how to keep the hatches shut and pressed against their sealing strips.
The photo also shows another mistake. It might be hard to see but the hatch sticks out about 10 or 11 mm in front of the bulkhead, just far enough to prevent the floor panels from lifting! I will have to cut a notch in the edge of the floor panel. Easy enough to do but then all the primer, undercoat a topcoat has to be redone. Alternately, I can just accept that the hatch cover has to be removed before the floor can be lifted.
Here are the two forward hatches. The blue tape labels are so I don't mix them up - they arent't interchangeable! Another mistake, the pins stick down and stop the hatches opening easily! I will shorten the pins a little but they will still be in the way.
Next photo shows the inside of one of the hatches with the two lugs that hold one edge in and the neoprene strip in place. Even putting the neoprene strip in provided an opportunity for a mistake! I put the strip hard in the corner between the two layers of ply. When I tried the hatch it would not close and eventually I realised the strip, in the corner had nowhere to go on one side so didn't compress as easily. Moving the strip out a couple of mm from the corner fixed this.
Now I just have to work out how to hold the other edge.