Spend the money for a good high end custom cover. There are several good manufacturers. I have an Aurora, and it has been great. It's starting to get to the point right now that the fabric will soak a little water into it. I plan to re-treat it in the next few weeks with the water-proofing chemicals. It was pretty much 100% water-proof for over 2-yrs.

Of course a roof over it would be better. But, a really good quality cover, that fits correctly will keep the water out.