As the heading suggests here are a few tips on using your coolbox effectively.
If you don't already have a coolbox and are planning on getting one then I suggest buying the biggest one you can afford but avoid buying the cheap supermarket ones because they are next to useless. They don't actually have any insulation in them just a pretend vaccuum. Coleman extreme or igloo are very good ones for example. They also make for good seating.
Forget using those blue iceblocks or bags of ice. Instead use frozen 500ml water bottles. Frozen water bottles melt a lot slower compared with using ice and you don't get water sloshing around your the bottom of your coolbox. When the water bottles are melted you then have icey cold water to drink.
Try to always completely fill your coolbox and open the lid as infrequently as possible. (air warms up quicker than matter or something like that)
Best tip is this:
"Pre-charge" your coolbox before using it.
About 2 to 3 days before the festival completely fill your coolbox with the frozen water bottles, close the lid and leave it.
The night before the festival take out about half the bottles and re-freeze. The remaining bottles should still be mostly frozen at this point and still capable of keeping your box very very cold inside.
In the morning just before you set off take out the bottles in the box and put to one side for another time or use them for the journey. Fill up your coolbox with your goodies and as many re-frozen water bottles as you can, obviously the more the better. Also it is obviously best if your goodies are also as cold as possible before putting in the box.
Using this method with a coleman extreme for me means I get ice cold drinks and water for 6 days (even when I end up cooling a six pack or 2 for neighbours too). Also if I need ice I simply break open a frozen water bottle and smash it up a bit.
Enjoy your fest to the best.
