HDSLRNOW.com shares this simple tip to provide shelter from the rain by using a small sheet of specialty waterproof Gore-Tex fabric and a few rubber bands to hold in place. I'm sure you can get lucky with a plastic bag, but Gore-tex is very light weight very durable type of fabric that can take a good amount of use. Not a bad thing to have in the bag when you're working outdoors. You can find this same Gore-Tex material via eBay (click here)
Gore-Tex WindProof WaterProof Nylon Fabric