It is very difficult to keep a slushy overnight because it is a mixture of water and ice. The freezer is the best option as long as it is either under 0° C. If you leave it in the fridge and the water gets frozen you will end up with a hard block of ice.
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A hydro flask can also be helpful. It helps keep the slushies to a closet currency consistency. Adding salt to your ice cube could lower the melting point of the ice cubes by several degrees. The ice cubes stay colder and long enough.
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