I wouldn't mess with all that. Instead, get a piece of cardboard and cut a curve in it while holding it horizontal, keep putting it up and trying until you get the shape right (as a template). They buy some 1x8 pine and transfer the template shape to the pine and cut it with a jig saw, then screw the bottom edge of the plastic to the pine. If it looks good you might take the pine back off and cut a curve around the back so you have about 3 inches of pine left; that way it shouldn't hang down too low in back. Or attach the back edge to a couple of hanger straps, even easier.
I wouldn't attach it right at the bottom edge, but maybe an inch or so up.
I have seen people try to fix broken motorcycle fairings (which are abs) using fiberglass on the backside. It rarely sticks properly.
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