After finally getting around to building my undertray, I got the two middle panels in. My biggest problem was space underneath the car, but I figured out a way to lift the jack higher.
Here are the two middle panels. you can see they're trimmed around the exhaust so it doesn't trap heat and/or fumes, or melt the coroplast.
Here's a picture of the front undertray I've had for a while.
There was a definite reduction in wind noise, as well as a jump in economy. My estimate is a 5 MPG gain on the highway. My daily 40-mile commute used 2/3 as much fuel than before.
Photos of the rear panel of the undertray to come, when it's finished. I'm still working on the diffuser fins.