[aerohead]

Any drag reduction is going to come from a pressure rise over attached flow, decelerating rearwards on a streamlined diminishing cross-section.
A Gurney flap can't do that.
A truncated Kamm-back is best.
Take a look at roofline 'silhouettes' Volkswagen chose for their :
- Volkswagen 'Flow-Body', L1, XL1
- Porsche 911 GT1, 918, Taycan.
- Audi E-Tron GT
- Bugatti Veyron, Chiron
You just chop the roof off like a loaf of bread, wherever the length is acceptable.