[SlowButSteady]

Using containers of frozen water, rather than loose ice, also has the advantage that it leaves you with bottles of ice-water for drinking. Also, I take all the items that can stand freezing and throw them into my home deep freeze a couple of days before a long trip, and keep everything out of the icebox that doesn't absolutely need to be in there (fruit, eggs, et cetera). That way, there is actually a lot more ice in the box at the start of a trip (in the bottles of ice AND in the frozen food), and relatively warm stuff that doesn't need to be kept cold for a few days doesn't add energy (heat) to the box.