: Anybody out there have any ideas?

Robert,
If the problem is a difference in the contour of the floors between the two boats; perhaps shape a thin piece of minicell with the bottom to match the floor of one yak and the top to match the bottom of your seat (which matchs the floor of your other yak). Like an insole in a shoe. As you may know a change in height will affect performance so I would carve your seat for the boat you use most often and use your "insert" for the other.
Good Luck,
Timothy Mulherin