Kristen-

Have you tried an Ice bag?

When I had to take Lucky to the vet that was 1 hour away, he got stressed out even though he was in a a/c car. The vet recommended that I take one of those bags that contains stuff that will freeze, but slowly. I did that, and put an old towel between the ice and cat.......he LOVED it! Kept him cool and calm. And when you have a calm kitty in the car for an hour, that is GOOD!!!!