As accident might have it I was in Kmart's garden section and just happened to spy their Midwest Garden Pad for $5.99... they are the much softer closed cell foam and about an inch thick: with a little creative engineering (read scissors and tape) I fitted them on my Tarpon 120 Phase III seat: now my butt is completely comfortable, the sitz bones do not bottom out, and I can fish in comfort for as long as I wish... all for the price of what a spool of fishing line would normally cost.
If some of you yak folks out there suffer a numb and painful butt in your alleged high end comfort yak seats, I encourage you to experiment with the gardener's seat pad... I could not be happier with mine and it only cost pennies. Better yet, the gardener's pad does not elevate you enough to foul up your center of gravity as your butt compresses the foam a bit. If for some reason you don't like them in your yak they are great to sit or kneel on when working around the house and you need some padding