Carbon tape is an ideal heat source. I took 3-foot of tape and put it in a strip of cloth. Soldered switch and radio frequency controller. Now I can smoothly adjust the temperature, turn the heater on full blast, if it is very cold or very windy. Turn on the smallest power, when I sit in a cold office.
Outdoors belt runs on 3 batteries 18650. in the office I turn it to the power supply from the laptop.
Run time from 1.5 to 10 hours, depending on the selected power.