I bought one of these sheds last year for $200. Moto-Shade.  10′ x 20′.  http://www.moto-shade.com/Product_Detail.aspx?Id=73

At that website they list stores where you can buy it. The UV breaks the plastic fabric down in less than a year, but tech there told me the trick is to get a new one under the 1 year warranty before the year is up. 

Its not big enough for a complete multihull, but many components can be built in one before the multi gets too big.