The guy with the wet cloth has it nailed
The latent heat effect is the saviour for all campers - more easily recognised as the cooling effect of evaporation. Each action has an opposite and equal reaction so the warming up of the wet cloths causes evaporation and a cooling effect.
Maker a simple camp fridge how ever small you need it. Polystyrene sides, top, base and door and soak in water. The inside will become cold. Just don't let it dry out. another way it to simply place a plate inside a muslin sack and hang it somewhere safe, wet of course and hey ho another camp fridge.
Play around with different things and you will soon get to protect your butter, bacon, cheese, etc,
Latent Heat Effect.....you can't beat it
Me I use olive oil instead of butter anyway!
