The other day I located a company here in Australia that does hitch mounted carriers, fully compliant and legal throughout the country, Link below, It's on the gallery page and the third picture with the large 4x4 wagon with the dresser/cabinet on the carrier is what I imagine my vehicle would look like:
HitchMate Cargo Carriers Gallery, Motorcycle Carriers Gallery
So if I was to create a "Tool Box" on the back, that just happened to conform with the stream lining template in the design, throw a couple of light tools in there & no more questions from officialdom, it can carry up to 160kg which is more than enough for my purposes, and I might actually carry something with it on occassion.
It comes back about 2 feet from the body, which would give me about 3 feet horizontally from the roof edge, this should be ample to get a significant Kamback/semi Boat Tail structure happening, I could also longer term close the gap between the vehicle and carry box with appropriately shaped foam strip so that it would be completely smooth in airflow transition.
I will start searching for all relevant threads tonight, but am fishing for any comments before I part with $$$$.
Thanks in Advance