For front/rear weight distribution. Or beer.
If I interpret that correctly it's a slant chop aft of the C-pillar. Doable but a lot of work.
Here's another alternative, the VW GTI W12-650:
It preserves the existing structure and adds essentially a bubble-top coupe inside it, with an inverted airfoil section for down-force. This sacrifices some interior space, but it could be reduced in size [and effectiveness] to some NACA ducts and tubing.
https://www.miataturbo.net/race-prep...e3/#post887810
The trick is you would be working with converged jets of air and where do you want those in the wake? Maybe the upper corners?
Given the body type and use case, I'd think an open boxed cavity.
