I took a liking to dropping a few of those cheap, oxygen activated hand warmers in the bottom of my fart sack when cold weather camping. If my feet are warm I am happy. 2 or 3 or those will add a lot more heat to the bottom of you bag than you would think. Not practical for the long term obviously, but certainly worth every penny when you can afford the space for a few.

I’ve tried using warm stones from the fire wrapped in a towel to great effect as well, but I am getting more efficient (lazy) the older I get and can’t be bothered with that nonsense anymore.