The car needs to breathe or you will trap moisture. I don't know how it would work outside but a local guy spreads charcoal briquets on the garage floor under his car. They act like a desiccant and seem to help keep vermin away. I've also heard of people using charcoal in the car. An open bag in the front and one in the back to keep the moisture in check. The pool cover underneath with some charcoal and don't completely seal it may be a good recipe.