Hitch receiver mount for aero mod?

[GasSavers_maximilian]

Drew up what I think is the shortest realistic tail I could make that'd do anything. Car would only be 15.5' with it on, with it adding just over 27" to total length. Need to do some more research to see how badly this is away from the ideal length/taper.

[GasSavers_maximilian]

To get the other extreme I used the same curve as one of the 1930s German vehicles on the aerocivic site. Clearly it suited their car better and that annoying faring at the top of my window makes things much worse. It would add 45" to my car's length, for a total of 17'. Still doable, but starting to get long. I figure I've now bracketed the answer, which is a start.