To be honest, go to your local hiking store or check online for a reflective blanket. Growing up, to save $ on electric bills, my folks put the reflector blankets under mattress pads. Think about it, our body heat is 98 degrees. The reflector blanket 'reflects' your body heat back to you with no need for 12V
or 120V. All of us were warm all winter, and at some point, the reflective blanket kept us too warm and had to be removed. Just my opinion, but one I'll use when boon-docking.
Thanks , Derek