I remember that New Beetle. The Cd went from higher to lower than stock.
I would call yours a Bonneville spoiler—0° angle for mimimum downforce. It provides a surface for turbulence/vortexes to reattach to. How did it compare to the 2nd iteration?
The base plate, as shown, is a vertical section of the template. I could easily do one with my bike rack. It looks like this:
It would be easy enough, but without instrumentation I would be trusting the seat of my pants.