Sorry I can't help with the question, as it's something I'm interested in too, but for different reasons.

However, since you did say "...which would be good for keeping your feet warm.", I wanted to suggest Boot Blankets. I have some and only have worn them a few times, but they do work.

They are light weight and slip over your boots. Some people just wear a few pairs of thick socks, slip their boots off, then just wear the blankets. Might be worth looking into since the heater is just for keeping your feet warm. See attached picture.

Good luck,

iSnipe