Empty box cavity like the big-rig trucks (laid on the side). Bonneville style spoiler and extended lower diffuser with side walls (terminate in LED taillights) to maybe 25% of the vehicle width. ~18" or less? Walls can be inset or break at the angle they use on hatchbacks. (30°?)
Base pressure reduction and a smaller wake. Test in coroplast. of course.
http://www2.warwick.ac.uk/fac/sci/wm...baseplates.pdf
This series also covers dynamically extensible base plates.