Perhaps I can add a little to this discussion since I made my own seat.
I am a firm believer in ergonomics. (That is the science of making things comfortable to use.) In making my seat I first made a mock-up out of 2" styrofoam sheets that are readily available from any home improvement store.
After finalizing the shape and fit for both the car and myself, I then made a wooden model of the framework.or base upon which the cushions were to be mounted.
I then proceeded to make that base or framework out of aluminum. Since I don't have the ability to weld aluminum, I either riveted or screwed the entire thing together. The reason that I went with aluminum rather than wood is because I used no wood in building the car and did not want to start using it now. However, if you would rather use wood, that is your choice and there is nothing wrong with that.
I made provisions for the seat to have seven mounting points. Two of them go to the frame and three of them go through the reinforced back of the body and the two remaining mounts are through the floor near the drive shaft tunnel.
I then got some proper foam from an upholster and glued that to some 1/4" plywood. Yes, I used wood here because it made it easier for the upholster to glue and staple the material to than if I used aluminum. I also used an electric knife and a 4" grinder with a sanding disc to shape the foam for the desired upholstery pattern.
The back rest is located with two pins that go into two holes in the piece of angle at its base.
The bottom of the seat is held in much the same manner as the back rest. There are two pins near the front corners of the seat and it also goes under the angle that is at the base of the back rest.
The cushions are easily removed for cleaning or "drying out" after one of our many summer afternoon showers down here in the Sunshine state.
Jim