Can we park a 25'metal shipping container, plaster the floor and walls, seal (waterproof) these surfaces and "build" a cast concrete top, to function as a water storage cistern for rain water off the roof? We need 12,000 gals storage and that's very expensive in plastic above ground tanks; a high water table and local inexperience in building underground storage makes this a risk. There is no piped supply available and the capacity of a shallow to supply all year round is doubtful. Rainfall information make its use very doable. Any suggestions?