Scott,
I recomend adding a couple of 12 volt fans that can be run by adding a extra wire or two to get the power to them.The ice idea is a good one but you do not want the plants or the pots to be directly on top of it.Remember Air flow. Humidity and of course light is what it takes.They would be much better served with less chance of loss if there was a way to transport them in the car on a seat wher the temperature extremes would not change as much or as drastically.
Robert
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