This is based on the "nICE Mug"
Simply put water inside the outer mold (Big cup), then place the inner mold inside the outer mold. (Best to do this over a sink). Make sure the mold is completely full.
Place this in the freezer until frozen. Allow the mug to loosen from the mold, and place inside the small cup. You should be able to get at least one drink out of the ice mug.
Commerially this can be purchased in a set of 4 for $50 plus shipping. At a unit cost of approximately $5, using high quality PETG.
This is a savings of $35 for a set of 4, and a savings of 65%.