Well, I'm a fiberglass novice, but I've done two seat so far and the first one I've put a lot of miles on already and no problems yet and about 20,000 miles on the seat.
Basically, all I did, and again I'm not telling you I know what I'm doing, is strip everything off the seat pan down to the plastic, then I cut some 3/4" Exterior grade plywood
to the shape and size I wanted the seat to be. I put some screws up from the seat pan into the wood to hold it in place then I simply started putting resin soaked
fiberglass cloth onto the plywood and wrapped it down around the bottom lip of the seat pan and up the inside about an inch. Several coats of resin and cloth later I
sanded it down, painted it all black with some spray paint just to make it look uniform and then started gluing the seat foam down with some specialty contact adhesive. I
used a hot knife to cut the bottom of the foam out around any humps or bumps that I wanted to smooth out or just to make it fit tightly to. Finally, I shaped it to fit my
fanny with a small surform file (rasp) that has a curved face so I could kinda "scoop" out the foam where I wanted to go down and I rounded the sharp edges off with it
as well. After several trial fits where I placed the seat back on the bike and sat on it trying to discern any pressure points I wanted to eliminate I finally arrived at the
shape I wanted and took it to the upholstery shop. Probably the only drawback to the fiberglass is that it makes it more difficult to secure the vinyl to. The shop tried
to staple the first seat I did, but the fiberglass was so hard the staples wouldn't go in and it jammed up his staple gun, so he screwed this one on and riveted the last
one.
Again, I'm not trained or experienced with fiberglass so others may have better information for you and I may have used more hardener than I should which might
have caused it to be more difficult for the upholstery shop to staple into, I don't know. It seems to bond to the plastic seat pan without any problems I can find
though, so that part was good.
Good luck with your seat.
