Houston won't get too cold, but it might get too hot. In which case, insulation will still be important.

Buy a few styrofoam coolers, they can be found nearly anywhere. Add one or two ventilation holes in each cooler. Snake bag them, and put a few in each cooler depending how big they are, add crumpled newspaper or paper towels for padding. Run a probed thermometer to each cooler. Place the coolers on your back seat, run the thermometer somewhere that you can easily see it.