Sharing this today because I just made one. I found 90% Isopropyl Alcohol at Dollar General for $1, the higher the % the slushier if will be. I also used gallon bags and 3 sponges inside to give it some structure. They were $1 as well. The box of gallon Ziploc bags was $1.50.
Homemade Ice Pack – A Flexible and Reusable Solution