I have a cover for my Kamado (WSM stored in garage) made out of Sunbrella Marine exterior material. It's awesome! Here's some info on the material: color fast, breathable, resistant to sun, mold, mildew, atmospheric chemicals, salt, rain, etc. and comes from the manufacturer with a 10 year warranty (excluding my labor). The best warranty in the industry. Sunbrella is the same material you see used in the marinas on 80% of the boats/sail covers. Seams are double sewn waterproof style, seam edges turned under and sewn. Heavy 18oz. 'Shelter-Rite' nylon reinforced vinyl (truck tarp) is sewn inside around the mid section (6" wide band), and inside the very top to protect from rubbing against top of hat(damper), hinge, etc.. There's also a matching sunbrella handle sewn flat into the top; comes in handy for handling the covers during install/removal and can also be used to hang the cover while not in use. You also have thread color choice; matching color, black, white, or whatever color you want.
Don't know if the guy has ever done any covers for WSM's but I'd be happy to contact him and ask if anyone is interested.