Here is a simple, comfortable seat you can quickly build from plywood leftover from your boatbuilding project. This is the seat, after experimenting with other seating solutions, that I put in all my boats. And I know something about seating: see my other business venture.
The key issue in boats like the ones I design is the driver is sitting at the same level as the throttle. This puts the driver's knees higher than the hips, which tends to concentrate weight on the tailbone (coccyx). Believe me, bouncing around for twenty laps on your tailbone leads to nothing good. I learned this in my first race -- and was glad we were not racing again the next day. I needed time to heal.
I moved next to padded seats. But I soon realized that leg support was needed. I put a riser under the front edge of my padded seat, and that was an improvement. Later, on a new boat, I built the original Dillon Seat.
This seat puts the driver in a semi-reclined posture. Its curved bottom and tilted position evenly support thighs and buttocks. No padding is needed. I put them in all my boats.
You can build one of these seats with a small amount of 5mm or 6mm exterior- or marine-grade plywood, and a very small amount of lumber. A handful of screws and some glue hold it all together. A couple coats of paint, varnish or epoxy and you're done.
NOTE: This seat works very well in a boat with a narrow cockpit with high coamings which will contain the driver laterally. Also, this is NOT a free-standing seat. The back must be attached to a bulkhead or similar structure. It will NOT support a driver's weight on its own.
Plans include online construction notes and photographs to guide you through the project.