I constructed my front belly pan using my old air dam, 1 3'x4' coroplast sign, 1 18"x24" sign, 6' of landscaping edging (
Vigoro 20 ft. Landscape Edging Kit-3001-20HD at The Home Depot), 2 dozen 1/2" sheet metal screws and 6 12" zip ties.
I started by trimming the top and bottom off my old air dam leaving only the middle flat plastic portion. I used the same attachment points and screws to attach the old air dam to the valance and used the same method to attach it to the outside of the landscape edging. I attached the coroplast to the bottom side of the landscape edging using screws every 8" or so. I trimmed curved pieces out of the small coroplast sign to fill out the edges of the belly pan. I attached the back portion of the belly pan to the control arms and the front crossmember using whatever points i could find using zip ties.
That is it in a nutshell, it took me most all of an afternoon but I was figuring it all out as I went. If I did it again it would take an hour. It still needs a bit more work and reinforcement to make the coroplast stiffer but that will have to wait til next weekend.