Just thought that I would mention that I can keep my cutting mat in my car, even in the summertime, without fear of it warping.
I brought my cutting mat that I use frequently to the lumber yard/hardware store and found a piece of thin 1/4" plywood and asked that it be cut into 2 pieces the exact size as my cutting mat. I use the largest black binder clips that you can get at the dollar store or stationary store and sandwich the mat between the two boards and clip the boards closed on all four corners as well as a clip in the middle of the two longest sides with the giant binder clips.
This keeps my cutting mat flat and takes away the worry that your mat will warp in the car, no matter how hot it gets.