Keeping a freezer full is the biggest key to efficiency, as far as I
am concerned.. . . Just fill up some empty milk jugs with water and
keep that in the freezer until you need the space for food.
When I have empty chest freezer space I put in cardboard boxes, line
them with plastic bags, fill them with water and let it freeze. This
helps a lot with energy use and when the squirrels commit suicide on
the power lines, causing an outage.