Just a suggestion. It may have been mentione previously on another forum, but it is worth searching the discount bin at walmart. Every now and then they drop the price of their Ripstop nylon, DWR nylon )water resistant) and Silnylon (waterproof) fabric. If you're not in a rush, it might be worth a hunt.
A good suggestion, but in other forums with a high percentage of DIYers it's been noted that many Walmarts are discontinuing the bargain bin as well as the fabrics section. Great if the local one still has it, but not a guarantee.