If the idea is to keep air out from under the rear of the car (causes lift), install a V-patterned piece of sheet metal/plastic that hangs a few inches down from rocker panels in front of rear tires with the tip of the "V" around the trans tail shaft.

For reference, look at the aero skirts on a semi-truck trailer that hang down from the center front of the trailer and expand outward toward the tires at the back of the trailer.