Has anyone had experience
> fabricating a custom form fitting seat using aerosol urethane foam

I did it using a two-part foam. First I made a temporary "form" of plywood that sat in my cockpit. Then I lined it with visquene. I put a small piece of 1/2 inch closed-cell foam exactly where my buns would be. Next, I wrapped myself in saran wrap (I know, sounds Kinky), mixed up a batch of expanding foam and sat in the hardening mixture. Ten minutes later I had my form. To get out I had to literally cut my way out of the excess foam and saran wrap (Can you imagine how embarasing it would be to get stuck?) I did some carving and filling to compensate for the folds in the plastic and other irregularities, then covered the whole thing in fiberglass.

The resulting seat is butt ugly (Pun intended), but is far more comfortable than any seat that I'd previously used. The seat goes up my back to where it provides lumbar support, and under my legs to pick up my thighs. If you don't need or want the back so high, I've heard of people sitting in a tray of sand, then making a plaster of paris plug from that.