Here's what I do in central Florida. I have a giant cooler (the type and kind you see at tail gate parties) in my trunk. In it, I store my camera, lens, and film when I absolutely must. There are no ice or cooling agent in this cooler. I try to park in shade but this is not always possible.
Returning to it after few hours, the inside does get warm but never HOT. I have suffered no damage so far and I have been doing it for a while with both digital gears and analog gears.
The reason I don't put ice or any cooling agent is that if I cool significantly to the ambient air, when I take it out, water can condensate and fog, just like coming out of cool air conditioned room with glasses on. I reasoned, my goal is to keep it _not_too_much_ warmer than if I were to carry them with me. I have done this up to about 4 hours with good results.