I've been using fiberglass rigid seats in kayaks for a lot of years probably 8 or more boats. I make my own seats that are a copy of early Valley kayak seats. Tractor style with an upward slant.
I was given some press blankets from a printer years back and that gets glued down under the seat. Mostly on the back butt area where the seat touches. It would be similar to a rubber auto floor mat in thickness - hard rubber not spongy. Under the front of the seat I use closed cell foam. So really the back butt area is touching and the front is almost touching but with closed cell under it. The side lips of the seat are bolted in to the hip braces (cheek plates - whatever you want to call them). Mine are never suspended, they attach to the deck and run to the floor. The seat is solid and I never had a problem with any kind of distortion on any hull etc. S&G or Strip.
On the bolts that attach the side lip of the seat to the strut, I use a fender washer that I bend to the shape of the seat and a fender washer on the back of the strut. A fender washer is just a large washer with a tiny hole available at boating stores. I paint the washer black so you don't see it.
I'll post a pic or two - not on my main computer right now.