Hi,
The top and the bottom of my sketch are arcs, so the angle of change is constant. If the total angle has to be 15 degrees or less on the top, and 4 degrees or less on the bottom -- the thing would be nearly as long the xA itself. This one stick out ~4'-5" from the rear bumper, and the full boattail would be ~11'-0"!
I think in that case, that a full Kamm back (on top, sides and bottom) would be the only practical solution.
Yes, I already have small partial Kamm panels, and I think they do help.