20 dF inside a car, just get better clothes. $2-300 for some good insulated hunting or construction clothing that doesn't need fuel should handle those temps. Do you have to get out and walk rounds or anything, clothing helps there as well.
Sitting in a car a electric pad would be real easy to make. The toughest part about making a vest is getting good conduction of the heat into me. Putting it behind your back inside of you jacket you should get good conduction and not need many watts.
Or just buy something like this. 5v and 2am is only 10 watts, I think I'd want a little more.
Heated Vest Battery Electric Power Vest Motorcycle ATV Snowmobile Vest | eBay
Looks like most of the jump starters have 5 amp limit on the 12v outlet, so I'd build for 2-3 amps, 24-36 watts. 40 watt light bulb is a lot of heat if it spread over only a 12x12 area.
Maybe 2 amps would be a better goal, 24 watts, use a long enough wire to get 6 ohms of resistance with a 12 volt battery and you have it, hot melt glue that to a piece of cloth 12X24 but only on half of it, the other half will be folded over to hide all the wire and glue mess. Connect some 12 gauge wire to the 30 gauge heating element, glue the 12 gauge to the cloth really well, then fold the other half of the cloth and glue or sew the whole mess together. Add a fuse in the 12 gauge wires, switch is optional, I just disconnect when I don't need any more heat, depends where your battery is.
If you find that's not enough heat, just make another one. Just need a multi-meter, hot glue gun, some wire, and a piece of cloth.
Found 30 gauge normally has a 1 ohm/10' resistance, goes up to 1.1-1.2 over 100dF so I'd get 50', measure the resistance and see if close enough.