Make a patch that looks like a riveted gusset plate and add another on the driver side.
I think the wheelwells and floor-pan offer as much potential as the tail, and Cd will have suggestions; but there are
- fixes—like spoilers, vortex generators and Gurney flaps
- mods—Kamm-backs, skirts, spats and difussers
- boat tails—extend the overall length by the wheelbase and wonderful things happen to the Cd. If you can live with the aggravation.
The really exotic stuff includes Coanda nozzles and Plasma actuators.