I asked you to send me your email address, so I could send you photos. That was the last I heard from you. They can be built from fiberglass, but you can't duplicate the outside dimensions while retaining the 90 degree angles used on the original plastic seats. (There is a reason these plastic seats are now rare)
Here is a bike I did a few years ago for a build off contest.
Here is another Cat (Endura) I did using the same craptastic technology.
You need to have a frame and sissy bar present to build it. As you can see in my photo, I had to build a new sissy bar, so I mounted it beneath the seat and made the seat frame, sissy bar, and seat pad all mount together. It was very sturdy that way.
I'd trade you a very chromed out Powell for this. Time wise, I think I had about 300 hours in mine.
Supposedly some people here have original plastic pieces, but they will not let them go for duplication. They do hint at getting someone to 3D print, etc, but "I've got a juicy bone and you don't" is a common theme here. You cant ride anything with a plastic seat if you weigh more than 20 pounds anyway.