Inflattable car bubble
http://www.carcapsule.com/
I'm a bit worried if the vinyl can leave scratches because it has a contact with the car before inflating, otherwise I think it's a pretty good product for long storage.
Mine is $1500 from
http://www.cover.co.jp/
I know it's expensive for a cover, but it's the only cover for the boxs that is actually made domestically (made in Japan - I don't like made in China products for lots of reason).
It has 3 layered cover (water proof polyester, water proof with heat permeable film, soft linen). It fits nicely for the boxs, tough on the outside and very fluffy on the inside!