[freebeard]

It needs plan taper, so you might keep cutting. If you could remove those plastic windows and use an electrical heating tape to put a diagonal crease in it, and notch the fiberglass to bend it for the increased tumblehome then pull the lower corners in a bit — then it would conform to The Template.

Alternatively, cut the top of the window at 22° and put as much compound curve into the new construction as you can. Instead of fiberglassing over cardboard (unless it's an egg-crate there's lots of opportunity for grief) think about buying preformed fiberglass panels and just glassing the joins. Use thin H-channel to capture the edges, drill holes and sew it together first.

IMHO, of course.

wdb -- That doesn't look bad. Two fiberglass rods in an X and some vinyl-coated fabric for a boattail erected on the tailgate. Why not?