We bought our freezer secondhand about 10 years ago. I had heard that a freezer needs to be kept full to help it run more efficiently. To increase freezer efficiency, I started rinsing and refilling plastic 2-liter pop bottles with water and putting them at the bottom of the freezer. I leave room in the bottles for expansion so they won’t burst. I have almost 100 recycled pop bottles frozen, with lots of freezer room leftover for food.
Besides keeping the freezer from running so much, we now have ice packs for coolers and a supply of emergency water. When the power goes out, I throw a few of the bottles into the fridge just to be safe.