Don't overthink it.
Your just trying to fill the wake.
Mair suggests that you want a smooth curve to an included angle of 2x22=~45°, then truncate to whatever length you can live with. On the top and sides, with the bottom at 4°. That's why cars from the 30s looked like tail-draggers. Upstream air affects the result; your notchback roof and stock underside work against you.
The truncation is the same shape as a base plate, just filled in. Take the profile at the rear and just shrink it down and lower it a little. The truncation can be flat convex or concave, that won't make much difference.
So much for the shape.
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...me making one that looks good...
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As far as the materials, I suggested PolyMetal because it is a pre-finished material available in a variety of colors. I'm experimenting with edging it; routing away all but one skin and folding that over. It's hard to do on a table saw, the skin is .007" and my router needs a new collet chuck. I think the edge could be wrapped around a wire to stiffen it.
The redwood bender board (at your landscaping supply store) is light and it cleans up nicely if you want the Chris-Craft look—varnished or fiberglassed. With fiberglass, unless you lay a gelcoat insidie a female form there is a lot of finish work involved.
You may wind up with multiple pieces. It looks like the bike rack lifts with the trunk lid. A 2nd piece to overlay the bumper and partial bellypan? Clear covers for the tailights or trailer lights? Will there be holes when you remove that 'race car' spoiler?