Decided to bust out the jigsaw while the car was up on jackstands and the rear wheel was off, just rough-cut a half-circle of the diameter of the rear wheel arch, wanted to mock it up.
EDIT: There was a jackstand in front of the jack. I took the pressure off the jack but left it in place as a sort of paranoid second layer of safety while working.
Without the rear wheel in place, it looks pretty great. My long term plans would be to fabricate something with fiberglass that would be "reversible" or symmetrical. A sort of compromise, making something that sticks out as little as possible from the sides of the car to clear the tires, and can be molded TWICE, and installed once on each side of the car.
I have never made anything with fiberglass before, but it doesn't look especially difficult, just expensive and very time consuming.
I might also go the "metal support and ABS or coroplast" route, but I really want something that looks neater than corrugated plastic.
Here you can see it sitting over the top of the side with the tire still installed.
The rear of this is not aerodynamically pretty, but it's also not accounting for whatever taper I could squeeze in the limited space. Overall, I am satisfied with at least seeing what could be. Already planning more ideas for future mockups.
Addendum: I left the rear mud guards on as a possible mounting point for a flexible skirt to hang down below the planned fiberglass part. I want to close off as much of that wheel well as I possibly can. In the future, I may remove the trailing mud guards if that proves infeasible.