[Michael Moore]

Brian, you've got a lot of time and infrastructure constraints to deal with.

You can only simplify so far (unless you are amazingly lucky) and then you've got to put in the work to get what you want.

There is flexible architectural 1/4 round molding but it seems to go no bigger than 3/4" radius, and for the leading edge of the seat back/fairing I'd think you'd want something closer to 1.5-2". If it came in the larger size it would be very handy.

Here's an idea for you with a quick model that will hopefully make it clear. Get some flexible large OD hose -- maybe you can find shop vac hose with a fairly smooth outer surface, but you may need to get vinyl/rubber hose. Pin thhe hose (silver) to the front of your seat back/box (green) matching the edge of the box, fill in the side gap (red) (consider some of the thin aluminum roof flashing, it is very handy), do some bondo to get as clean of a plug as you can and then pull a quick fiberglass mold off of that.

You don't have much time available to cruise Ikea, Home Despot, random stores, junkyards etc looking for found objects shaped just the way you want. A friend did that many years ago, looking at plastic waste baskets/clothes hampers until he found one that was just the shape he wanted, and then used that as a mold for a race seat tail section.

cheers,
Michael