Instead of choosing from any number of perfectly easy, cheap and nicely shaped XR750 style seatpans out there that could probably just bolt onto my frame rails directly, I'm going to make this as tough as possible by rolling my own out of fiberglass/epoxy.
To wit, I'll shape the base of the seatpan directly on the frame rails. Here we see the patient draped with semi-sterile vellum paper (mind the resin in the carbs!). I've wrapped the rails with thick cardboard (~2mm) and what seemed like three rolls of packing tape. This will then get a good soaking of PVA and the glass/resin will go right onto it.
I'm reminded of just how hard it is to work with sticky substances like epoxy, fiberglass and tape. It's a huge pain in the ass and whatever you can do up front to be neat and tidy pays handsomely down the road, when your rubber gloves are sticking together, your pulse is thumping, and you know deep down inside, by the exothermic heat given off from the resin curing in the pot, that in 10 minutes this shit better be done or else it's another reject...now where ARE MY UBER SHARP JAPANESE SCISSORS!!!