Anyone try making their own? Found some discussion on MTBR on this, and Amazon has cheap small heating pads, like these:
https://www.amazon.com/Eachbid-Heati...GGYF556N0Y7GQZ
Someone on MTBR said they built theirs with cheap pads like the ones linked, which power off of USB plugs, so can be run off cell phone external batteries - so that's a source for fairly cheap and safe power.
Seems like this is doable for a lot less $ than the commercial ski marketed boot heaters.