Hi - I can picture what you're talking about
:
The challenge of
adding material to reduce drag is you potentially increase frontal area (A). Since total drag is the product of Cd * A, the added material would have to decrease the drag coefficient enough that the net result of Cd * A is a smaller number.
Another modification approach to encourage a cleaner flow transition from the wheel well & wheel onto the quarter panel of a "conventional" design: cut a pool noodle (or equivalent) along its length to get a 1/2 round shape, and attach it to the aft part of the wheel arch. Voila - instant generous radius/transition which the air should like better.
I hope you'll post about your Malibu MAXX. Now there's a vehicle well suited to a Kammback addition!