I hope this hasn't been covered before, here goes.
My brother has just brought a big box of freezer kits. (Designed for relief of sports injuries). They are a plastic bag with crystals in them, which when needed can be mixed i.e. shaken. This causes a chemical reaction inside the sealed bag and causes the contents to freeze down to an advertised -8C. It occurs to me that placed inside the cavity of an animal it would help greatly in lowering the temperature of the meat while in transit from stalking permission to home.
They retail at a few quid each, but if brought in a decent number come out at a pound each.