I don't know about the poly, but Tyvek is pretty noisy when the rain hits it. You can pick up some waterproof fabric that might set you back about $30. JoAnn sells some, so do other online fabric places (cheaper than JA), just do a search for Outdoor Fabric.
Life should not be a journey to the grave with the intent of arriving safely in a well preserved body, but to skid in broadside, thoroughly used up, worn out, and loudly proclaiming Wow! What a Ride!