After a few hours, when the water is frozen solid, add a little more water, but not too much, because you don't want the first layer to melt and dislodge from the bottom of the tub and float to the top.

Add water and let it freeze until the tub is completely full of ice. Dump the sand out of the bottles and carefully re-fill them with warm water. I recommend using a funnel, because any warm water which spills out onto the ice will hasten to ruin the ice block itself.