The speed that frozen water melts is determined by external temperatures and the ratio of the exposed surface area to the total ice volume. Raising the volume of ice while minimizing the increase in exposed surface area and reducing the direct contact with elevated temperatures will extend the amount of time that ice cubes will stay frozen. Slow down the melting of ice cubes by creating an insulated environment that eliminates exposed surfaces to stay colder for longer.
Pack the ice cubes into the cooler or plastic tub as tightly as possible until it is completely filled. Shaking the ice container side to side will often settle the cubes at the bottom to make room for more at the top.
Cover the exposed upper surface of the ice cubes with a couple of layers of plastic wrap.
Seal the cooler or plastic tub securely with the lid and cover the container with a large towel or blanket.
Place the covered container in a cool indoor or shaded area to avoid the heat of the sun.
Open the cooler or plastic tub and remove the plastic covering only when you are ready to use the ice.