I use gallon jugs that previously held tea (not milk) and freeze them solid (provide air space so the water can expand without damaging the plastic)... they sit side by side well, and when melted you have water to drink...

for the 2004 hurricane season, I used 4 frozen jugs in a 5 day cooler (I should have pre chilled the cooler but didn't, but do now)... I did not open the cooler as I worked out of an older one, at 7 days I had access to ice at school... I checked on the cooler and the jugs still had some frozen ice... approximately 1/2 cubic foot of meat, originally frozen had thawed but was still usable, so it did add to the original thermal mass...