I normally use 2 ltr bottles of frozen water to keep my box cold. If you put salt into the water before it freezes, this will delay the thawing process enabling your box to keep cooler longer
Thanks for that informative reply 100_mph_ pennine. I have not tried using blocks of ice because I could not be bothered drying the melted water off everything. Your point about surface area is something that had not occurred to me, but, with this in mind, (and disregarding the effect of the number of times the box is opened and warm air is let in - which is going to be variable from one person to another) it would seem that the gel packs (having a larger surface area) are, in fact doing a better job than the frozen bottles. Similarly I suppose that two separate 1pint bottles of frozen milk would be better that one 2 pint bottle, or four 500 ml frozen bottles of water better than one 2 litre bottle.
Am I understanding this properly?
I guess in practice I will carry on with what I am used to doing, which is to put in a couple of 2 litre bottles, put my food & drink in, and then fill up any available remaining space with the gel packs.
I have used gel packs, used packs which you have to hydrate. However I keep going back the frozen bottles.,
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I too found 100mph-pennine's reply re surface area very informative - thank you! - I think it explains why lots of ice cubes and freezer packs works for me (if I understood correctly).
Coolboxes UK (who sell Yeti/iceytek coolers) also give useful advice as to to best way of keeping cool boxes cooler...
http://www.coolboxesuk.com/usage/
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I'm going to freeze a good few Capri Suns. I froze one to try it out (to ensure pack didnt split) and it was fine. Put it in my sons bag for school trip and it was still frozen by end of day in the hot sun and only a frozen water bottle in his bag also to keep lunch cool.