I made a plywood form in a shape that suited my body. The actual frame that I used was an
image of my profile, so it was the shape of the top surface of the seat.
It was two pieces of plywood, with about 10 cross-pieces of scrape 2 x 2.
I "sewed" the 1/4" foam to the cross-pieces with dental floss, then two plies of glass over what would become the underside of the seat. The dental floss pulls right out after it is done. After cure, this becomes fairly rigid, but the shape can be adjusted a bit since it only has glass on one side.
The frame is made from square wall tubing, 1 inch square, with mounting plates on each end. They fit low to the floor on the back spar, and high on the front spar. That is, the bottom spar cap in the rear, and the top spar cap in the front. The tubing has a "dog-leg" shape. It is parallel to the floor from the rear spar, forward about 10 inches, them jogs up to the front. There are two under each side.
The seats have a piece of 3/4" pine glassed to the bottom, with an attachment made from the same
square tubing. The seat attachments were fabricated and the pine was cut and fit.
Next, the seat was glued to the pieces of pine which were on the seat frames. This put the previously glassed side on the pine and left the un-glassed side up and exposed. Two plies of glass went on next, with the seat in the plane.
The seat was removed from the plane, trimmed, and the push rod cover/arm rest was installed.
The raw edges of the seat have about 3/8" of foam removed from between the two pieces of glass. This cavity was filled with uni-directional glass and flox.
The wooden reinforcement was left over from an earlier idea. I was going to have a hole in the
center of the seat to access the baggage area. I changed my mind and made the top fold down.
I put duct tape in the places where the reinforcing structure was NOT to be attached to the seat top, glued more foam in place, shaped it, and the covered with glass. The epoxy does not stick to the tape.
After it cured, the seat top was cut off
Recesses where cut and glassed for the hinges.