OK, well from last night it looks like this is going to be good enough; I tried to photograph it but it's hard to make out. Here's a 3/4 profile from below where you can see the strips of fiberglass that were first formed around the frame rails, and then while that was wet, I "Nested" the seatpan onto them, allowing a bond to occur.
This is how it looks ejected this morning. I think it'll work out okay once it's cleaned up with a dremmel. Might add some more reinforcement in the gaps where the frame and seatpan deviate. I can also adjust the height/orientation of the seatpan off the rails by varying the thickness of the rubber pads used when they are applied, but for now it's "close enough"
As an added bennie, this assembly is much lighter than the original, and the license/light holders are integrated as opposed to bolt-on: