Both.
I just made my insoles. All I did was take a dollar store windshield heat/sun shield, traced my feet, and cut them out. Its just a thin piece of foam with a layer of foil. Will try tommorrow. (watch it not be cold). I also thought wrapping an existing insole in mylar might work. Also pulled an extra anti-perspirant stick out of the closet, and might hit my feet with it...not sure. That might be a good idea just in the fact that you can get those small travel sticks at the drug store. Again, this is little things that take up no space that you could use to give your feet an extra edge in cold climate in a survival/outdoors scenario. Don't get hung up on the example I gave (cold feet at work)...that was just what got me thinking about it.
The good thing is, if this windshield reflector trick helps, you can make a whole lot of pairs of insoles for a dollar from one of those car shields.
After that I'll try the mylar since that is something most people have in their pack. If not you should....check ebay. You can get them from medical suppliers...about 12 of them for $8 shipped. The more you order, the bigger the discount. Way cheaper than the camping aisle.
Edit to add: With the money you save, you can buy a new Becker.