The ice cube trays can be bought in different sizes that is what you need.I have just made some that are 1/2 inch in diameter and l also have the one inch and 3/4 inch size.If you drill out the holes a bit in the top it makes it a lot easier and you don't get the air gaps in the tops of the balls.Don't forget to use thick rubber bands to hold the two parts of the trays together or the soap will leak.