http://www.lady-natalya.info/insight/aero-06.png
Modified that panel again. It looks like the length of the tab on the left side of the picture (front of panel) should be 2.5" instead of 2", so the entire panel should be 20.5" long.
What happened was I did an oil change and noticed my aluminum belly panel (immediately in front of this one) was fitting very tightly. Turns out it had warped the brackets it affixes to on the bottom of the radiator. I had those installed backwards, which was biasing the aluminum panel backward on the car, which meant I made this coroplast panel about half an inch too short.
After fixing the rad brackets and putting back the aluminum panel, the coroplast one was wont to pop out when driving because it was a little too loose. I fixed mine with extra wire and two zip ties and putting two more screws in the back, but I think that leaving it a little longer should do the trick for anyone else who makes one.