Interesting challenge, innit?
The wheel well arch is really rigidly defined, no rolling the trailing edge like a gen1 Insight. Maybe some vortex generators to carry the air over the wheelwell gap. Apparently that's what these are supposed to do
If you tie a skirt to the steering linkage you can use the eye for the steering dampner in the tie rod and if the lever arms are longer than the steering the skirts can move 1/2 as far as the wheel as they turn.
The Volkhart-Sagitta has a frontal area of 2.1sq meters compared to the Beetle's 1.8sq meters, with half the Cd. Bubble skirts might not be so bad.
That axle beam is available narrowed 2-4" as a service or replacement part, or you could adjust the wheel width and/or tire section, e.g. 145-15 on 3" rims.
Or a conveyor belt lip that is sized so when the wheel turns it contacts the tread instead of the sidewall?