Cider is okay warm more so than beer anyway.
I never bothered fighting it at all the festivals I have been to. Just keep it in some shade under a gazebo and put it just inside your tent when your not there so it dont get stolen. Sometimes I have used cool box with a few ice packs which keeps it semi cool. Anther idea could be take a bucket of water and fill it with water from the water point when you are gonna have a few cans and chill them in the cold water (its always really cold no matter what festival) then empty when you are raedy to move out and watch some bands.