Best I have found: a Pea Pod or Polar Pod. Just get the vent hole down reasonably small and all of the air surrounding your face and head will be MUCH warmer than the outside air, though not as warm as below the neck if you are using either clothing or a light top quilt neck collar style and to fill in gaps and prevent drafts. It is one of the commercial pods strongest points IMO. You can close it down if needed or to as small as a quarter or dime, or even all the way. You can, of course, if needed also be wearing all of your normal head/face protection inside of the Pea Pod. Or, you can lay on your side in a narrow hammock like a Claytor No Net, face on edge of hammock, and the Pod twisted so that the breathing vent is slightly below your face. Cold air will not be very good at rising to enter that vent.
See pics in post #6 here: