You can emulate the 777 car and freeze a few giant blocks of ice well in advance of the weekend (I use disposable freezer containers, about 3x4x5"). They won't cool as much as small ice cubes, but will stay frozen a lot longer.
I had the 13 quart cooler when I raced the P-car and it stayed cold through an Enduro. The "medical grade" coolers are better insulated and don't leak water.
Here's a thread to build your own.