There used to be squareish plastic freezer containers in the stores. (I don't know if there still are since I no longer have a freezer) They were slightly larger at the top than at the bottom and the square shape would fit in the freezer better than rounds. The trouble with jars is that things expand as they freeze and would break the jars. That neck for the cap would get hung up and get pushed off. I have seen small glass freezer jars where the cap (non screw on I think) is the widest part of the jar.,