60 cm of 15mm carbon heater tape from www.carbonheater.us and now on Amazon placed and sewed on each of some factory insoles. Carbon tape is thin, very soft but strong enough. Because of big length it provide uniform heating of feet.
Tape connected in parallel 2x30cm.
It's powered by 1x18650 for each one. 3,7V x 1,3A almost 5W.
Carbon tape covered by a thin cloth.
Warm and very comfortable. I use it with my heated vest and gloves.