Take another look at Compugeek’s data. There is a factor you are missing. Time!

People have used thermal mass of heavy and thick walls to delay the effects of temperature swings between night and day. The hot day temperatures slowly heats up of the heavy mass walls keeping the interior cool until the onset of night. At night, the cool night temperature reverses the process, cooling the exterior until the interior cooled off. This delayed the hot temperatures until night time. The mass delayed the interiors heat cycle. To keep the interior relatively cool all the time, the windows and vents would be opened at night. It also works the same way with insulation, i.e. the foam cooler.

If you would look at his temperature table again, you will see the average difference between his night time exterior temperature and his inside cooler temperature is between 21 degrees and 22 degrees. I bet these low temperatures were also at night. If he had opened the lid and cracked the trunk during the night, he may have kept his interior cooler temperature closer to his goal of 75 degrees. This opening could be accomplished with manual labor, thermal ram, or motor. All of which is less complicated, less expensive, and more efficient than electrical cooling.