Jeff, a plastic bag is normally all I use. If the trip home is more then 2 hours I'll bring an small Igloo cooler with me just to play it safe.
If I know the ride home is 1+ hours, I'll ask the store to pack extra water for the coral/fish. I'll also keep the plastic bag out of direct sunlit when riding in the car, the floor normally works well I cover the plastic bag with a towel.