If the exhaust is anything like on a BWS then turn your charcoal chimney upside down and foil the top. The smoke will come out the chimney holes but keep the rain out.
For homemade, a metal can with holes cut in the sides, flipped upside down over the chimney. For future use you can go to the hardware store and look through the air ducting. There are several pieces that will work there in several diameters.