The energy savings from keeping extra ice in the freezer don’t come from how well water holds its temperature (thermal mass) as much as from the fact that less cold air is lost when the freezer is opened. The energy gained through the walls of the freezer (thus the work the freezer must do to maintain temperature) is a function of the quality of insulation and the temperature difference between the outside and inside - the thermal mass is not involved when the system is kept closed and the temperature is maintained. So if you’re frequently taking out these ice bottles to make room you’ll probably end up using more energy freezing them than you gain from having them in there. The most important tip for saving energy is to set the thermostat so that it maintains a temperature that is “just cold enough” to keep the temperature difference minimal. The same holds for cooling your house in the summer (or “just warm enough” in the winter).