[Dozer523]

Here is another good cold weather tip. Reschedule your bowel movements for the evening, most folks like to do it in the morning. You don't want to expend body heat keeping a big piece of waste warm overnight. You will sleep more comfortably. There was a great WWII movie I saw once, where the Germans snuck up on the Brits in the desert in the middle of the night. Everyone was ready just before dawn. The German Commander ordered his force to hold fire until the Brits went to "take the morning constiutional" then they let them have it! The attack did not take long. As for me, personally, that is not the way I want my body found. I try to be a "tea-time" guy!
Here is one I've read about but never tried. High altitude mountaineers take a hot water bottle and pee in it at night. Then hold the bottle between your feet, or where ever is coldest, I guess. The fluid will initially be almost 98.6 degrees. The technique might also make it so they don't have to get out of the bag to take care of that. I would imagine that when the liquid begins to get cool you could put it outside the bag, but I would not want it to freeze solid. Again, no personal experience.
As for the larger boot size don't try to get warmth at the expense of blisters. You can still walk on cold feet and be comfortable. Blistered AND cold feet, no thanks.