Yes—You're getting most of the Template.*
...except the most important thing—it's a half-body of revolution in section, IOW a half circle sitting on the ground. You've got a square. Consider these alternatives to the half-zepplin:
You are at A and you want to get to F. There are interesting alternatives for a hitch mount at G and H. No vortexes is better.
The payback for the added length is linear. 10% gets you 10% and so on. 10% gets you 5mpg. Or something; but a 100% boattail
might get you 100% better mileage—40 to 80mpg is believable—but it's a lot of pain to drag one around, as you've seen. So you pick a percentage.
*What you've called "affected zone" is probably less meaningful than Fineness Ratio—the ration of length to maximum camber, which I think of as at 30%/70% of length. Nothing much happen before the point of stagnation: think about how water wraps around the bow of a ship.
Edit: These shapes aren't sliced with a ground plane, but should give the same Cd for a given cros-section.
The biggest difference is the rounded shapes have better cross-wind performance.