try a 'roho' for arse support, they are air filled cushions designed for people in wheelchairs to prevent pressure sores. they are expensive if you buy from a shop ($600+) but you can get cheap ones from ebay if your lucky. you can either sit them on top of the existing seat (if set up properly they wont sit you any higher at all) or replace the aircraft seat all together (yes I expect to be lectured) but the difference is amazing, it reduces fatigue to the point they should be compulsory.
ok lets have it