The standard steel shipping container has steel sides, roof and ends. It has a plywood floor which is attached to steel cross-bars. If you want to make a container higer, remove the plywood and cut the cross-bars, then mount the container on a stub wall, or use the cross-bars to lift the whole thing up.
If you don't need secure locking doors that is the end of the matter. If you do, then weld steel plate to extend the door down to the floor level and then add a hasp and padlock.
Simples!
