With the plate/spoiler angled up to intersect "the template" with a shorter overhang of the rear of the car as shown in the diagram, I have to wonder just how much additional drag will be created.
Out of curiosity, try moving "the template" forward. I think you will find the result interesting. I think, but it is just my opinion that "the template" misplaces the rear because of the choice of both the airfoil and the position of the leading edge, but that is just my opinion.
If you take a look at the
current Subaru WRX five door which has an aero add-on at the top of the rear window and compare it with the
four door notch back I think you will notice that there is a trend toward establishing a sharper separation point which basically creates a Kamm back on two box designs. The wing on the notchback is probably more of a styling feature to look like the actual rally cars than something which actually does something beneficial at normal highway speeds, but who knows. It is all a matter of drag and few of us have the capability to quantify it on different model designs.
I have seen ads for add-on affairs for SUVs which have a narrow slot to allow air to get to the underside of the wing at the top of the vehicle which were supposed to help keep the rear window clear and improve the air flow in general. As with all such things, a fair degree of skepticism is probably in order.
Chevy has been running ads for its Equinox, which uses a similar approach, claiming that it has the same Cd as a Corvette ZO6. (The truth is that they are both about 0.34-0.36 which isn't all that wonderful, but sounds nice and besides it is an improvement for a SUV.) One or the other of their ads even talks about the Kamm back aerodynamic treatment. Once again, it is as much marketing as anything else, but there is some truth to it.
The Subaru with the trunk mounted wing does illustrate how you could mount some end plates on the trunk lid and position things pretty much however you wanted. The ones on the Bonneville cars are more in line with what I would think to be low drag designs though.
Cheers