In an easy-pour cup, combine three quarters of a cup of water with one quarter cup of rubbing alcohol. Pour the mixture into one of the Ziploc bag. Carefully squeeze the air out and seal it. Place that Ziploc bag into the second Ziploc bag, carefully squeezing the air out and sealing it. Then place the whole thing in the freezer until needed.
When you notice the mixture leaking out of the inside bag, you can easily transfer it into a new set of Ziploc bags.
I have done this for a long time. They conform to most body parts, are small enough to tuck away in a corner of the freezer and are cheap to make.