[Christ]

In all of those applications you've pictured, they're using it to control speed-wobble by creating downforce.

In the application you're looking for, it won't do much for aero, but it will help keep dust off your rear window, and might shade it in the early afternoon.

It works so well on the New Beetle because of the design of the car.. it creates a fake "slope" between the edge of the spoiler and the upper (trailing) edge of the roof, which approximates a better angle for aero than what was available by the car's design. In your case, not so much, unless you use it like a Kamm-Extension.