I finally decided to install the hear hatch. After putting it in, I realized what a convenience it would have been when I went camping. In any case, lack of a Rear Hatch might have been a negative selling point.
I also decided that I wouldn't use foam bulkheads on the next build, so that gave me some 3" minicell to play with. I bought an angle grinder & carved this out. I tried it for about 1/2 hour, and it didn't seem "right", so I whittled it down some more, and it still doesn't seem great.
When I tried Jay's Outer Island, it had a custom-molded fiberglass seat. Absolutely no padding, but the most comfortable seat I'd yet sat in. I'm going to try that next.
I Tried making a mold of my butt in cement. 1st attempt was not auspicious, because I (stupidly) got concrete instead of cement. I used a great deal of Plaster of Paris to try & smooth it out. Then I tried again with mortar , making the mix looser & wetter than everyone recommends. That one seemed more like a "mold".
I also did a little more whittling on the foam seat, and gave it a one-hour trial today. It was better than I thought-didn't feel nice and cushy, but after 2 half-hour paddles, no numb legs!