Cool boxes

or ice — and its insulation thickness and how often you open it are what matters. An electric one maintains the temperature without ice, but it needs a power supply and draws current in a car even after the engine is turned off, so keep an eye on your car battery. Choose the volume based on the number of days and people, not what fits in your boot; a half-empty cool box retains cold worse than a full one. Prepare your ice packs frozen in advance.