Fill the egg carton with suet or your homemade filling.
Put down some newspaper to protect your table, because some filling might ooze out of the holes during this step.
Fill each cup to the brim, and even to overflowing. Then close the egg carton just like when it contains eggs.
Get two heavy duty elastics, like the kind they use to put brocolli stalks together. Have your child hold the egg carton vertically while you stretch the elastic over the top and down between some of the bumps. Do this at the other end as well. This holds it together.