We'd suggest not covering it, as the very action of fitting and removing covers can lead to marring if the car isn't 100% clean. Why not get a wool duster for light dusting down on days when you want to take it out and the level of dust accumulated is light...
Wool Duster - Polished Bliss Car Care and Detailing Products
...and just wet wash it when you've left it for longer and a thicker dust coat needs to be removed? Far safer way forward in our humble opinion.