Well, after a good night's rest and a fresh look at the tuft testing results today, I've decided it's good enough as-is. I'm not going to change it.
EDIT: Neil, I might just test that wheel strake/boattail idea to reduce turbulence, too. If that doesn't help, when I make the permanent tail, I'll use a little less curvature in the sides, which will add about 20 cm / 4 in. to the width of the rear.
Tuft testing video: (used Youtube's "image stabilization" feature, which is fantastic! The raw video is pretty bumpy at times.)
Next task: A-B-A test and weather-proof it. Weigh it, measure it, to answer those inevitable questions.