I made frame A in a similar way as frame E. I do overlapping joinery at the joints of the slats. Frame A is a watertight bulkhead. So it is closed with a 8mm Okoume plywood sheet. I add 10mm in the thickness of the douglas fir slats (50mm instead of 40mm) for reinforcement reason in this area of the cockpit floor while the construction is heavy loaded from the main sheet traveller. In addition it will help when the crew is jumping around in the cockpit. I spent approx. 30 hours of work.

Frame A while assembling on the workbench in my basement workshop.