I recommend checking out the print details as well.
I made this for a friend's upcoming 21st birthday, but I figured I should share the fun. I haven't gotten to test this version yet, but I will update this as soon as I get a chance to do so. UPDATE: It worked perfectly. I cannot express how important it is to remove the layer lines (I sanded to 600 grit). It released from the mold very easily with only a couple minutes outside the freezer and a little warm water over the mold.
Put the two halves of the mold together and twist 90 degrees to keep the top part from floating out of place. Add water through the opening between the molds nearly to the top. Leave a little room for expansion (I haven't figured out exactly how much space yet). Freeze. Allow to sit out for a couple minutes after freezing. Twist the two halves of the mold and carefully remove your new shot glass. (The glass in the picture I posted is actually from a previous version, but it used the same dimensions for the cavity. This updated model should be able to produce slightly taller glasses)
After printing, I recommend sanding the inside of the bottom mold and the outside of the top mold until there are no more layer lines. This should help it break free once frozen. Be patient with removing the glass from the mold. Sometimes it can take a while to break free. If it is especially stuck, run the outside of the mold under warm water or put some warm water in the cavity in the top part of the mold.
Since Fusion and Cura dont agree on the axes and sometimes the dimensions, I've included pictures of the recommended printing orientation with dimensions. For me, this kept the inner surfaces of the mold as smooth as possible.
I'm considering making a version with thinner walls, especially on the bottom part, to add flexibility and remove insulation to help the ice break free faster. Comment below if you would be interested, or if you have any other suggestions. I'm happy to hear feedback.