Since I was reasonably certain that the rear "shell" was an ok idea but really, REALLY poorly executed, I did the only rational thing. I hacked most of it off! I probably cut way too much off initially, and went from no taper (or outwards flare) to too much taper. It's pretty easy to build it a little higher with foam, but it's a serious PITA to hack up fiberglass sheet and reshape it. Here's the first test, you can see a somewhat reasonable taper top to bottom.
Unfortunately the zero-taper-top and the heavily tapered sides caused some serious vortex action near the top of the sides:
My MPG stayed steady at ~13.7 with this taper, even with the horrible vortex. I had a spare backup camera so I installed it on an aluminum arm so I could see how bad it was.