My first customer...
He reported a significant pain at the top of the back after 4 hours.

If by "top of the back" you meant "near the top of the backrest", it could simply be needing some adjustment. My first boat put my back near the top of the backrest in agony after about an hour. I tried moving things back, and back some more. Finally I moved the seat forward less than an inch from the original position, which changed the angle of the seat back and it made a huge difference. If my interpretation of your description is correct, he has pain in the area where the backrest top is applying to much pressure and increasing the backrest angle will reduce pressure at that point.

From my interpretation of your picture the seat and backrest look very flat. The seat bottom in my opinion should be carved about 2" deeper than the sides and the carved width should only be about 12" wide (even for my fat arse). It will seem awfully thin when making it, but that's what works for my butt.