I was actually looking at a Fiberglass front bumper cover for my car that extended a few inches lower. I would them smooth over all of the pointless holes to make it aero friendly and have a sturdy decent looking piece.
Car already has nice side skits with 'spats' to break up the flow for the rear wheels.
Then fill the side 2 vents entirely, and make something that looks like a grill for the middle 'opening'. (Solid, maybe Plexi with silver stripes to look like a billet?)