I pre-chill the box with old plastic water bottles filled with tap water, frozen and then put in the Icey Tek as many days in advance as possible, or more likely, when I remember. The more frozen bottles you can get in the better, to really get it cold.
Then I pre-freeze as much as possible, burgers, sausages, bacon etc.
Then when packing I swap the bottles of old water for cold freezer blocks, and a couple of frozen ice cube bags, and a gel mat thing (from sainsbury;s), and another bottle of frozen water.
I took my Icey tek away on Fri lunch time, and when I brought it home Sun eve the white wine was still icy cold, the water still mostly frozen, and that was after a day left in a hot car in the sun yesterday. (but I did put the sleeping bags on top).
I am disciplined about not leaving it open, or opening it uneccessarily all day.