I'm finishing my 6th - all with a fiberglass seats. I make what Nick calls cheek plates, except mine travel from the deck to the floor of the kayak and are epoxied in place with fillets. They are positioned so my fiberglass seat (from my own mold) fits snuggly between them.Then I simply drill a hole through the seat and through the side plates and fasten the seat with a stainless 1/4-20 flat head bolt and lock washer. I use fender washers on both sides. On the seat side, I bend the washer in the vice to have a curve that matches the seat. I also make a rubber washer that goes between the seat and the plate and also under the finish washer on the other side so it doesn't dig into the wood. My plates are made from 1/4 birch ply.

Under the seat I glue on some rubber where it's going to make contact with the boat floor to avoid any abrasion. Under the front of the seat is closed cell foam so the seat rests against that. Pretty simple

Behind the side plates, you can store a pump etc. if your boat is wide enough.