Storing my car outside for extended period of time

I'm planning on storing my car (NC mx-5) outside for an unspecified amount of time, and am looking for the best way to do it to protect the car. It will not be driven at all during this time. If it is washed thoroughly before hand with a coat of spray-on ceramic wax (Griots 3-in-1), will it be clean enough to safely cover the car without worrying about scratches?

I've heard that car covers do more damage than good because they trap dirt and when the wind hits the cover the dirt scratches the car. Will this be a complete non-factor if I wash the car beforehand? Also, I've gotten the suggestion to use moving blankets in between the cover and the car as an added layer of protection for the paint. Is that a stupid idea?

If the cover is not recommendable, what should I do to protect the car as much as possible while it is being stored outside? Any and all insight is highly appreciated.