Several museums have had special canvus-type covers made to fit over outside stored cars. These covers are made up in a big "cross" shape to fit the car. Then the corners being laced shut. When finished it looks like a giant toaster cover.
I am aware of both Pennsylvania Trolley Museum (Washington, PA) and Orange Empire Railway Museum (Perris, CA) using these. In fact, Penn Trolley started this and OERM buys its covers from the same firm in the Pittsburg area that Penn Trolley uses.