Your search term within Ecomodder could be
'hitch mount'. Most of the implementations of your proposal fail in the same way yours would. (sorry). If you envision the airflow over the aft of the vehicle you would see vortexes erupting from the point of separation triggered by things like longitudinal edges and trim. This creates a wake bubble that will completely enclose the construction you propose. The most you get is wake filling.
The way you could make that work (and it would) would be to remove the roof completely and extend it out to the sides with a taped or molded transition. The taillights and license
must change to accommodate the airflow.
The reason Bonneville spoilers work it that they provide a surface for the turbulence streaming off the roofline to reattach. If you push the Kammback/boat tail concept to it's conclusion you get something like the Citroen Tubik:
