I have been looking at building a custom molded fiberglass seat and can't seem to find any reference to how these type of seats are commonly mounted to the boat.

Two methods I can think of are:
1) Make blocks of wood with blind nuts installed from the back side. Epoxy these blocks to the hull at the locations were the seat would be bolted to the hull. The bolts would be near the top of the seat sides and would bolt to the hull just under the coaming.

2) Mount the seat to quality urethane foam and shape foam to the hull. Fiberglass the foam for durability. Use velcro between the hull and the bottom of the seat. This would allow for easy seat position adjustment.

Any other ideas?