It can be done, but it's going to take quite a bit of steel to do it. I'd guess you would need heavy vertical steel beams every 4'-5' and then at least 2 horizontal beams to run the length of the container on each side, one end and the roof. Even then, you may still see some inward bowing of the walls/roof when you bury it. Having cut and modified a container, I can tell you the sides and roof aren't very strong. A round tank laid horizontally would be much better.