Mine sits outside as well and we have snow and ice to deal with, so, I had to develop a way to keep that canvas cover from caving in. It wasn't easy, I've refined it over the years, here is the final version:
https://www.dropbox.com/s/fplh6p6z0q3aw ... er%202.JPGhttps://www.dropbox.com/s/1hnh7jv13t2lv ... 0cover.JPGI had to come up with a way to secure the cross bows (fiberglass and very strong). I made wood panels up with the brackets used to hold those bows in place. This cover did not cave in this winter and we had a LOT of snow.
If you want the best, and don't want to build what I did, get yours from Fisher's Canvas in NJ. They are the best all weather covers period. The standard ones like we all have, are fine if you don't get snow. If you get snow, they will cave in and leak water in the interior, unless you do what I did.