PVC banner material is often used as a covering for this sort of stuff, you could also use a truck curtain side material but not sure if it has another name lol. banner material is available at different weights (thicknesses) and at different widths. How are you going to manufacture the covers?? You really need an industrial type sewing machine if that's how you intend to fabricate them. Sailmaker might be able to do it for you if you know of any.