I was trying to trace out the shape of the frame onto sheet metal when I realized there's a way faster and easier way to dimension things. Card and scissors. Easy to bend and trim into shape.
So the basic concept is to make a flat pan for the bottom and a "backrest" for the back. The foam will fill it in. Then the material will wrap over the edges and be affixed to the underside. To not see the ugly underneath, the edges should be curled so the material can be stretched under the lip and affixed out of sight.

So with the paper outline it was easy to transfer into my two source pieces. Flat 14-gauge steel for the bottom (shown already into the bending process), and a piece of fender for the back (so it automatically fits the curve of the fender). I used the angle grinder to cut the shape out of both. (I'm getting pretty good at cutting clean smooth curves with it--at least through thin material where it's easier to round curves)
Shaping the sheet metal wasn't done with magic tools like a mallet and anvil and an english wheel. Ingredient 1: Vice. Ingredient 2: Bare hands with a little weight behind them. The trick is little bends at a time, especially when trying to fold over a curved lip. I'd bite the vice down, give a little shove, unclamp, move down the edge a little bit, repeat.
Little adjustment with constant checks to see how I was doing... The second image I used a hammer to fold over a little lip in the nose.
With the base fitting neatly, I welded on the backrest part, and then rolled a small lip over for it as well. The grinder was used to blend the pieces together on the edges (the only part that really matters, the rest will be covered with foam). I did a lot of little tweaks to try to keep a small gap all the way around the seat pan to leave room for the vinyl to tuck under without interfering with the fit against the frame.
Last step was to punch a pair of holes in the seat pan to serve as mounting points. Then I drilled matching holes in the frame's seat-area gusset. Slide a bolt through each hole and weld them to the inside of the pan. Then I ground the bolt heads down. So now the seat just drops straight onto the frame and a pair of nuts tightened up from underneath holds it in place.
Finally a quick grind, primer, and spraypaint black so it won't rust. Then off to get foam glued to it and the whole thing wrapped in vinyl. There's a seat place across the street from where I live and that's where I ended up taking it.

So fibreglass is a better seatpan material, but I didn't want to learn that skill on this project. There's just two problems with making a seatpan out of steel: 1. It's pretty heavy. I don't really mind that though. 2. Upholsterers can't staple the material to steel. One place I inquired said I'd have to put a wood trim around the inside to give him something to staple to. The other place said it would be a pain in the ass, but they could do it with glue. (Though they ended up using rivets, looks like the glue wasn't working).

email: mikebike@rocketcar.net