[Big Dave]

5-6" is close. Drive it over some railroad grade crossings and what is left will tell you what will work. My guess is that 8" is probably A-OK.

I got my 3/8" ripstop conveyor belt from an industrial supply house in Naptown.

It is not a bit cheap. An air dam and two side skirts set me back $100. You are doing this right in one respect. Run one made of cheaper stuff and use it as a pattern for the more durable material. The ripstop conveyor belt is darned near indestructible, but the flip side is that durability makes it a bear to fabricate.

Take your "pattern" and pay the shop rate to get a good professional product that will last for years. Save yourself the agony of sawing/drilling this stuff. Ask me how I know it is hard to work.

Another piece of advice from having done this: clamp the front and rear edges firmly. Your skirt will try to vibrate like an oboe/bassoon double reed. If not clamped firmly even ripstop will wear through .