Alex84,
What they said. We started with a 60lt ice box, which was higher than length and width. We used a 10 ltr spring water bottle (squarer,flatter sided the better) that was filled 1ltr a day and frozen each time, which stops the sides bulging. We take frozen casseroles/stews in used chinese take-away containers. Meat is cryovaced and shaped into flat packs and frozen individually. The idea is to get all of the frozen stuff as low as possible with as little air space as possible, so every time you open the esky the air rushing in doesn't get to the bottom and need re-cooling. Prepare your ice now, the longer it is frozen, the longer it lasts.
We place the esky on the ground as it is cold and put other stuff around it to reduce any air movement.
As others have said with drinks, or put in a mesh bag and dropped in the creek.
Good food preparation should see you through the week, so you have more time to enjoy your stay, or deal with other issues like weather or mechanical etc.
But you will be in good hands in the group.