A few thoughts, are you currently bracing your feet at the top of the plywood bulkhead? If so you can add a cross piece or a large foam block to allow you to brace further down. I just rest my heels on the skin at the bottom of the boat.
Do you have any sort of backband? I've found that adding one that sits really low and supports the base of the spine made a world of difference to my leg pain. I am using the hip belt off of an ALICE pack which you can get from any army surplus store. You could also do this with your seat, build up the back similar to the ones on a rowing shell.
Another option to look at is to add padding to the bottom of the massik, to lock your thighs in better so you don't have to tense as much to keep locked into the boat.