You'll want to follow the streamlining template as closely as possible. It is below posted by another ecomodder on
another thread.
Your bug has too steep of a rear angle. To reduce your rear drag you have to add to the rear of the bug. VGs etc. will *maybe* help a bit, but there is no guarantee, and it could never match what a boattail is capable of. Anything that is not a boattail is merely a compromise, as the stream lining template is the most aerodynamic shape possible for a car with a flat bottom.
Have you read on the aerocivic yet?