Does anyone
: have any other ideas?

Here's a link to an item on the seating we use in our VolksKayaks. Been using it ever since I built the RightWind in '01, and find it simple and comfortable. We'll sometimes be in the boats for a full afternoon with no sign of numbness or tingling, just the usual stiffness from sitting so long in one basic position.

I did, on one occasion, forget the pad back at the cabin; used an old flat-panel PFD instead, and boy, was it bad! After less than two hours on the water, I literally couldn't get out of the kayak - both legs totally asleep, pins and needles, and all that. Since then, the 3/4 Thermarest is high on the "Got it!" checklist.

http://www.nessmuking.com/articles/the-simplest-of-seats/