Military shipping containers make great storage, but your readers should be aware that the large plastic cases are not necessarily waterproof if they have a pressure relief valve. Most of them have this due to the need for pressure equalization, e.g. atmospheric changes during aircraft transport. If you are going to bury such a case or use it in a wet environment, the small round valve can allow water to enter the case. Some of them are good at keeping water out for a short time, but if something is going to be stored for possibly years, anything that lets air in is also going to let water in. Your best bet is to seal the valve, so nothing gets through.