Not that I'm biased or anything, but the single biggest thing you can do to drop resistance is lower the car.

The underside is already relatively smooth. The only area to smooth out is the tunnel that hides the exhaust and driveshaft, and closing that in would turn the area into a furnace.

The Corvette boasts an amazingly low coefficient of drag, and it's a result of very low ride height and a small frontal area. We can't really reduce the height of our car's face, but a bumper that extends out a slight amount at the bottom would create more of a wedge than a wall.

All I know is since I've lowered the car, I've gained a consistent 2mpg. Can't be imagining that.