I started work on fabrication tonight (of the "good", permanent version).
I'm planning to glass directly over the upper cardboard panel because it's exactly the right shape already.
I removed the two "side" panels though - they were less than great (corrugated stuff doesn't do compound curves nicely). I recessed the panels about an inch or so to serve as a backing, and will fill the area with Great Stuff expanding foam, then shape it and glass it with the top section.
(The cavity that will be filled with foam. The tin foil is the "release agent" from the hatchback surface.)
I have it on good authority the foam is fairly easy to work. I filled one of the sides about an hour ago - hopefully it cures hard, because even though it's tack free, it seems pretty
squishy.
I would say this, so far: based on the amount of time I think it'll take to do foam/fiberglass construction, I think if you know your plastic and can get a proper sheet of ABS (or whatever is the right type to heat form into compound curves) , that would be the way to go.
Once the upper surface is glassed, I'll remove it from the car and add foam to the center section & glass the entire bottom, to make a sandwich.