The best material I have found so far is a type of memory foam used in wheel chair seats, aircraft seats and some pillows. I am using a one inch thick piece for my butt and it is more than adequate. The only drawback is that it is open cell foam and needs to be encased in a bag of some kind. I am working on a T-shaped bag with three openings to insert hip, back, and butt pads.

The most important thing I know about seats is that the closer your butt is to the botom of the boat the better, this is especially true for a boat with low initial stability. An inch makes a big difference.