Half ice, half water

 

Fill half a plastic bottle with water, turn it on its side and freeze it overnight. If you freeze it upright, the ice pushes out the bottom of the bottle, making it difficult to place the bottle on any surface. Take out the bottle the next day and top up with water.

How effective?  Essentially, this method made a very large ice cube in the water container. I used a 1.5-litre mineral water bottle and the ice took a few hours to melt. It was probably because I was indoors. Taking it outdoors will cause the ice to melt more quickly.