For about twelve years the ranger station on San Miguel Island was two 20 foot shipping containers placed next to a wooden deck. They were outfitted with windows and doors by a commercial company and airlifted in place. Not a very aesthetic setup, but I spent more than one stormy night inside, snug, warm, and dry, and extremely grateful for the shelter. They are inherently rodent proof, unless you leave the door open. That should be a major consideration for any storage unit.