Ive got some ideas on building a seat, be happy to compare. After seeing one apart the pans are pretty crude, and the hardware rivets on.
Width will be the factor, most arent comfy for adults.
I was going to stack 2 2x8's together and cut to rough shape and use a belt sander to create some dome. Router the edges round, or use belt sander. This will give the "buck" for hammer form.
Stack a thin sheet and a heavier sheet of metal for the inner and outer pan. Drill 2 bolt holes thru the wood and metal, recess the buck a little under the bolts.
Was going to use sheetmetal from appliances.
Start hammering the heck out of it. May need to add recessed areas on the back sides of the buck with a drill for clamp points to work the sides over.
Shorter sides will be easier. Lots of clamps. Trim the edges even with a grinder once its formed.
Weld or jb weld 2-3 studs to clamp the top and bottom pans together, weld or rivet the seat mount on, could even use rails.
Just an idea so far, but ive done some hammer forming making panels for auto restoration. The seat pans just need to hold shape and clamp together, dont need to be beautiful.