I second leaving off the Kammback. Much to my surprise, this post :
http://ecomodder.com/forum/showthrea...ibu-20326.html
Showed a possible
increase in drag with a Kammback, and a sizable drop from a NASCAR style spoiler instead ! ( Aerodynamics are so unpredictable )
How much money are you willing to spend ? If you are willing to flip the cargo carrier the " right " way and add some coroplast to it, you could create a nice virtual boattail in a few hours time.
http://ecomodder.com/forum/showthrea...g-17911-5.html
And if you haven't already, of coarse cover the " parachute " under the rear bumper as everyone is suggesting.