I liked the drumroll.
Okay, seriously, for what car are we thinking here? The Beetle/Super Beetle has at least four rear window configurations. Then, there are Karmann Ghias, Type 3 Fastbacks, ...
I'm also wondering from the standpoint of function. Using the New Beetle as an example, there are two main spoilers used. One is the electric one above (or if you prefer "in front of") the rear glass; the other is a static, crescent-looking contraption below the rear window. I have to wonder how effective the latter is despite its larger size.
I bring this up because the rear window on any of the Beetles is lower than the roofline to begin with. I'd be more concerned the accessory did nothing at all than it pushing in the window.