A sheet is a good idea. My garage is dusty as can be, but I was always concerned with scratches and tend to live with the dust until I can foam and wash as usual.California Car Cover products have always worked well for me.
Otherwise, old cotton bed sheets work too if you're looking for temporary dust protection.
I also am looking for a cover that keeps the dust off, but more importantly, keeps the dings out of the side of her... THAT'S my priority...