Removing all the clamps to the top skin was interesting but uneventful. That much curve causes a lot of strain in a piece of plywood. I guess I used the right kind and amounts. Everything held beautifully.
I drilled a hole in the fan space for the router bit and was kind of amazed at how well it cut a beautiful square. Some things just work. One side needed glue ; the clamps helped and squeezed the connection shut.
I used the same bit on the hatch overhang and with a little coping saw work got a nice clean edge over which the covering and plastic hinge will be placed.
I am very grateful for the blogs and Pinterest links that I have used a LOT!! In one link a man chided most builders for making the hatch build so hard with carefully arched vertical beams. He suggested that continuing the horizontal beam system in the hatch that most builders use is easier, faster, and looks just as good. I agree.
I bought 2 1x8s for the sides, shaped them to match. WAY easier. I cut clean 1x2s as cross beams.
I used the cutaway part of the walls to mark out the pair of 1x8s which must match exactly the 4′ curve of the galley end. Instead of using a slow jig saw, I opted to raise my skill saw blade and use that. It really worked well. Lots of sanding commenced.