When I need to keep something cold I fill some 20oz coca cola bottles with water and freeze them. They are strong, do not leak, and are big enough to stay frozen for a while. If you drink soda they're free, best part is that if you get tired of lugging them around you can throw them away. When you fill them with water, just leave some air at the top and I usually squeeze them a little while I put the lid on them so they have a little room to expand in the freezer.