You are a heater; a really good one. Better clothes will help you store and distribute that heat. If you aren’t wearing a wool base layer, it will be the single greatest change you can make.

Milwaukee is a tool company. Try Carhartt, they actually know how to make clothes.

Do you also have (I assume) nice quality base layers, maybe wool?

Catalytic hand warmers (they run on lighter fuel) also still exist fwiw.

I'm hoping you get some good answers. I can't wear long sleeves in my unheated machine shop (long sleeves + lathe = Very Bad™), and a 20 amp breaker can't run a lathe and an electric heater simultaneously. I've got a diesel heater out there, but it's just not enough to keep a drafty cinderblock building warm. I get pretty chilly out there, and some kind of battery heated vest would go a long way.

I had a vest from these guys for when id motorcycle in winter on the west coast.

https://www.ventureheat.com/en-ca

I recently got a vest from Ororo. I haven’t used it too much yet but I’m enjoying having the option of heat so far.

Got my sister in law a heated vest from Beston for a trip, and she was so happy with it. She wears it constantly during winter.