[lunarhighway]

if you're looking for an easy mod i'd experiment with a lip spoiler, most cars of this type have them these days.
also small ridges to detatch airflow at rounded corners are beginning to appear

if you want to go all the way get a second hatch and graft on a boattail extention or build something that attached to the hatch, hinges

it's a nice idea, but i'm affraight it won't work in the real world