Take plastic bottles of appropriate sizes and shapes (square sided ones pack well - use milk or fruit juice ones) and almost fill with saturated salt water - swimming pool salt is very cheap. Leave space at the top for expansion and freeze upright for a week or so. They will stay cold longer than ordinary water, but you can't drink it, of course. Mark as "SALT WATER - Do Not Drink" for safety.Once you have calculated the correct amount of liquid, you could glue the bottles closed, excluding all air, they will shrink when thawed. ABout a cup per litre is right.