[Big Dave]

I use 3/8" ripstop conveyor belting.

My air dam is about 24" wide and is partially supported by a couple of aluminum plates.

Expensive. Hard to work. Nearly indestructible. Impervious to heat, cold, gasoline, road kill, curbs, etc. Had mine on the truck for five years now.

When I have to replace it (who knows when? - it looks A-OK) next time I'll make a paper pattern, pay the shop rate and have it sheared and punched by a pro.

Grade clearance is an issue I'm afraid only a wind tunnel can give you an optimum answer. For a while I had full-width 3" ground clearance. Looked dorky and ground everytime I stopped but gave the best MPG. Now I have 6" grade clearance in front of the tires and 10" between (just covers the steering. Looks better, never grinds and doesn't give as good MPG.