I got a coolbox from Go Outdoors, and I've used it in the boot to keep food and drinks chilled.
I have got the 12V socket in the boot so it doesn't have any cables trailing over the back seats.
It works really well, providing you chill it down first (ice packs help with this). It will cool from ambient, but takes some time.
I brought a whole load of chilled food back from France and it was all still chilled when we got home.
works on mains as well (built in).
It will also keep things hot. But not at the same time as keeping thinks cold.
Always reminds me of the thermos flask joke; keeps cold things cold and hot things hot.
"so what have you got in it?"
"Two choc ices and a coffee."