Okay, Here's a simple proof of concept for the "Lost Foam" method:
I started with a rough shape built from layers of styrofoam bonded together from the epoxy resin that I've been using, mixed with a little glass microspheres so that it's more readily sandable next to the foam (reduces the resin's density so that it's closer to the soft foam):
Here's the rough shape, covered in packing tape and with a little handle bonded into where the fuel cap would go.
Cutting Glass from paper templates derived from fitting to the piece.
A lot of awkward steps later, the resin and fiberglass have been applied, then a layer of peelply, and breather cloth. Assembly shoved into a vacuum bag and sucked tight:
After resin cured, it's time cut away the peel ply and breather cloth. The exercise is much like shucking a very tenacious corn cob
Pour a little acetone into the cap area and let chemistry do its thing. once all that melts you can pull the tape out of the system. Placenta?
And here's the proof. Not terribly bad. Would take more than a little fairing to make nice and if this test were real, I'd probably use at least 4 more layers of glass;