Piper I feel your pain. I have been searching for a solution also.
The problem is two fold, sitting low with the knees straight, and staying in one position too long.
My last boat and chair were built to address this. It is more stable than most pirogues or kayaks and allows me to sit higher (14" to 15"). The new chair I built has a concave seat that takes preasure off the back of the legs and allows me to straighten them when I want to paddle harded.
I would think a 14" seat woud be too high in your boat and would not work well for serious cruising. Changing seating height often during the day has helped me a lot in my pirogue. You could build a base like this:
1 st. option 8.75"
2 ed. option 10.75"
Your seat is similer to mine and is at this height:
Normal height aprox. 5"
Add a boat cussion and you get 3 more options.
beekeeper