Oh, the deli cup with frozen water isnt to keep it cooler in the event of a power outage. It's there with the screw(in my case) on top of the frozen water so i know if my freezer loses power or fails when I'm not home. That way i will know it thawed out the rodents because the screw will not be on top of the frozen water in the deli cup, it will be sunk in the bottom, either refrozen or in liquid state.
I do know what you mean though about filling up as much of the open space in the freezer with frozen stuff so that the freezer doesnt have to run as much and in the case of an outage, it will take longer to thaw. My power was out for just over 4 hours and when it came back on, i opened my freezer to check and everything including the deli cup was still frozen solid.