It's a bigger upfront cost, but will last for many years. Griffolyn is a reinforced, UV- and fire-protected material used on truck tarps and things like that. They can cut a custom size and add grommets as needed. I've had mine for quite a few years (at least 6 I think) and it's held up quite well. I do have one small tear along a seam which is why I have a small white tarp under that area, in the pic I posted. Reef Industries can give you a quote:

http://www.reefindustries.com/division.php?div=1

As an alternative, could you use two white tarps, joined at a common transverse covering frame?

Max