A refrigerator needs less electricity to cool objects than it does to cool air. A full fridge is cheaper to run. On this theory, I fill my freezer with containers of water. When I first tried it some years ago, the total electric bill went down. Many covered containers will do as long as they leave room for the ice to expand.
Rectangular containers can be more tightly packed.
Really big rectangular yard-sale Tupperware works well. Quart juice boxes can be arranged in many ways. They bulge unless I fill them in stages and brace them on both sides 'til frozen. Round deli containers and any plastic milk jugs can work. Paper milk containers break.