Well, the side taper is now significantly less than before. About half as much. It's now slightly less of an angle than the curvature/angle on the side of the rear bumper itself.
The pic below makes it appear the tail is now as wide as the car, but it's not. That's just the camera. I added 12 inches / 30 cm to the overall width of the "transom".
I also added a little spoiler to raise the top surface closer to template, and hopefully promote cleaner separation.
Thanks for the advice & encouragement, everyone. I was ready to stuff the whole thing in the recycle bin yesterday.
Re-test to come!