I'm surprised no one has suggested contacting a couple of local commercial roofing contractors and asking if they have an offcut of membrane roofing material (often EPDM, but there are other materials out there) of suitable size from one of their jobs that you could buy. There are some really superb fabric reinforced materials out there that are often used as reservoir covers, etc. Google "membrane roofing" and look for local contacts. I personally have used three reinforced composite materials: butylene, EPDM, and Teflon (like the roof on the Georgia Dome). Good luck.