The common wisdom is that a lot of empty space (air) in a fridge/freezer is bad for efficiency. The reason is because every time you open the door, the cold air flows out and gets replaced by room-temperature air, which then needs to be cooled back down.
If you’re not opening and closing the door frequently, it’s not an issue. And even if you are, for a short-term solution, I would suggest that plastic bags filled with air would work pretty well, and be more efficient than bottles of frozen water.