I just use a California dust cover. It came with the car, but that's what I would have bought if it hadn't.
If you don't need a weather proof cover, it's best not to use one. You don't want to restrict air flow and trap moisture against the paint, you just want to keep dust off. If you're going to leave the car out in weather some of the time, it's not a bad idea to get two covers.
Democracy is two wolves and a lamb voting on what to have for lunch...