There is, however, enough there for me to make a plaster mould of it and then build up a fibreglass pan instead.
Certainly don't want to discourage your f'glas seat pan idea, but keep in mind the stress placed on the seat pan's hinge attach points, the latch assembly and the attach points for the rubber bumpers. Your photos show some metal cracking that probably began before the heaviest attack of the
rust zombies.
That's the original seat cover you have there, too. Note the number of pop rivets and sheet metal screws that pass through the thin, lower flange of the seat base and close to the edge. A fiberglas flange will have to be considerably thicker than the OEM metal. As such, it might be necessary to actually fab a wider seat base to provide clearance with the aft frame members. Both seats on my R65s pass close to the chassis when the seat opens up.
No doubt you could do it up in 'glass. As Bengt Phorqs asks me when we strike off on one of our motorcycle adventures,
"Mongo, what's the worst that could happen?"