Jo-Ann's and Walmart carry that "rubberized" fabric that is used for baby diapers covers and changing pads--and you could use a solid. Or, what about tarp? I saw an article in a quilting magazine about a woman that uses different colored tarps(I've only noticed blue, but she seems to have found many different colors) to make up the top and probably the back to. just used a bigger needle and thicker thread.