Canopies like this don't do too well in the wind and 1-2" of snow will stretch them to the breaking point (trust me)
http://moto-shade.com/PRODUCT_IMAGES/MS/Large/XL.jpg
I've had one for a while and have replaced the top at least 3 times now - about every other year. Does fine in the rain but deteriorates in the sun and wind.
I can't recall the name, but Newty was talking about a heavy duty cover that goes clear over the tower. Even saw it on his boat one time. It's got a zippered door up on the side that makes it easier for the thieves to get in and out of too.