Here is a great link that will help you understand how to store long-term food storage in food grade plastic buckets by adding dry ice. The LDS church recommends that you NOT stack 5 or 6-gallon plastic buckets more than 3 high as the lids can crack over time. Also do not store them directly on the floor. They need to be at least 1/2″ off the floor to allow air to circulate. Go here for the handout, and click on Plastic Buckets. You will also find other storage information.