[Otto]

The lawn edging with the PVC pipe insertion may be adequate stiffness. By heating the PVC pipe with a heat gun, you can bend it around a radius to match the bumper. That way, you'd have both a cheap and light application, but also the aero advantage of the radiused ends of the air dam. Maybe an elliptical shape would be attractive, as well as efficient.

Further, the arc-shaped device could be hinged at its tips (sides), such that it could be raised and lowered, up high for parking lot and driveway clearance, and down low on the highway, or anyplace in between for measuring optimal height. (Imagine the hinges on a motorcycle helmet visor, which it would somewhat resemble.) This thing could be retracted behind the stock bumper.

Use the same PVC pipe-in-edging idea to make side skirts between front and back wheels. That should use up the whole roll, more or less.

Another idea you could easily incorporate, is Hatchtastic's air dam/undertray mod, see his post on that.

Just add a sheet of Coroplast to the bottom edge of the air dam chin, extended back to, say, the oil pan.