I continued down the side, boxing in the doors and keeping the contours nice and smooth. The build is various thicknesses of styrofoam, glued together with Loctite PL 3X Premium urethane adhesive. I tried 3 or 4 different types, and the generic construction adhesive was by far the best. Most others won't cure with 2 solid surfaces, or have good initial grab but then fall apart 5 minutes later, etc. This urethane is a moisture cure, so if you are gluing two non-porous surfaces (foam and fiberglass sheet) you just spritz a light mist of water on it, slap it together and tape it down.
Here's the door buildup:
And the fiberglass sheet skin over the foam buildup to the rear wheel.
And the finished, rough sanded side skirt!
I've had to do some touchup where the tire rubbed, but otherwise the side skirt was responsible for a 1mpg improvement, from ~13.5mpg to 14.6mpg. The worst tank of gas was during construction, where I had a DTM-looking square cutoff behind the front wheel. 13.1mpg, ouch! One good side effect of the layers of foam is excellent sound damping of road noise, it's incredibly quiet now!