I built my seat out of a stock seat pan. I have an F model bike with the Duck tail hind end, so I think it lends itself a little better to the process, but a K model seat can be made to work as well
here's my process:
Strip down the stock seat. (Don't throw anything away... yet)
Mount the seat pan back on the bike.
Use your choice of material to form the design that you want -
I used Hardware cloth to form the "hump." It's galvanized to prevent corrosion, forms easily, and gives a good sturdy base for the next step.
Fiberglass over the form. -
This is where I like the hardware cloth: The cloth gives the fiberglass a skeleton, so to speak. As I do this, it all gets formed into one, big unit. If I were to do it again, I think I would make the hump removable.
Couple extra layers of fiberglass -
I used three, but more could be put on - especially if you don't use a Skeleton like I did, you'll want more layers for strength.
I also used a strip of thin sheet metal on the end to 1.) smooth over the edge, and 2.) I was going with the Snap look and wanted something stronger to put a snap on. It was a major pain in the ass, but I made it work at the end of the day.
Bondo
I bought Marine-Grade vinyl for my seat. Sews easy. less care needed than leather. Personal choice. -
Cut out your patterns and stitch together.
If your stock foam is okay - cut it down to make your new seat pad.
Paint
Hope this helps at least a little.
I'll try to put together a little slide show of the process.
