I think I have a better idea: The sign you put in your window should read:
"This is a Mexican hoosepoctlta hound. Bred for the extreme heat of the Sonora desert, he cannot survive in temperatures colder than 100° and is happiest at 130°. He becomes uncomfortable when the mercury goes over 150°. For his safety and well-being, please do not let any air colder than 115° into this car. Several of Pancho Villa's lieutenants had hoospoctla hounds, who would happily run alongside their owners' horses for 12 hours under the full sun at 120° needing only half a liter of water a day."
Of course you actually turn on dog mode and keep the car at 72°.
If there are gawkers when you come back to the car you can tell them it's a joke and show them that it's actually nice and cool inside.