I carry an emergency blanket at a minimum:
SOL Emergency Blanket Sometimes a more substantial Space Blanket:
Heavy Duty Emergency Blanket Usually I also have a heavy contractor bag. These items ore really not that bulky.
These things work very well for a desert shelter and for signaling. You also have to understand that I am not trying to travel light with only a carry on. My trips are usually 2 weeks long at least and involve a mix of outdoor activities and meetings in more civilized surroundings. I am usually checking two large bags regardless. My carry on is usually some reading material, electronics and my basic essentials for a surprise night out. They come in handy for layovers in airports also.