Thanks. You could:
- replace the flexible front air dam with a diverter that extends forward to the stagnation point. Currently air that hits the front bumper winds up underneath. A diverter divides the air and lowers the stagnation point. Extreme examples on time attack cars.
- Then the front wheel wells can have air curtains or spats (both?).
- The sidewall curtains could be doubled up (see Aerocivic)
- The rear fender skirts it needs could extend TurnpikeCruiser-style to an established rear separation point.
- How do you feel about working with fiberglass? You could pie-cut the rear top. Cut it into strips and 'glass them back together for the proper curve.
Of course, a full boat tail fixes everything. How much do you want to drag around behind it (parking, visibility, etc.)?