: I have used the micro balloons/epoxy slury to fill the pores of other foams
: with great sucess. The reason was so the resin did not fill the pores and
: add unecessary weight.

the other thing to possibly do is to mask the foam w/ some plastic wrap lightly spray glued on( test 1st) and lay up on the back of that. when set up, remove and contact cement on.

if variablility is desired, slot the back and use webbing to the side posts or cheekplates and bungee back to back of coaming, and loose hinge to the seat portionl so can bend or be allowed to tip forward if desired. or maybe even to seat sides itself.

don't make the seatback higher than the coaming, depends on clearance and specific thickness so may be best if below coaming, don't want to restrict too much movement.