Ah, here we go!
90 watts, $25.
http://m.ebay.com/itm/Indus-Tool-Electric-Foot-Warmer-Mat-Black-/282047969222
Or 135w, $60
https://www.matsmatsmats.com/commercial-industrial/heated/heated-foot-warmer-mat.html
I think I'll get several. Above the floor is nice, bec if they break they are easy to replace. And some could go in or on the walls.
Maybe I'll get fancy and wire them into a panel of switches -- adjust the temp by flipping on more, or fewer.
When on battery I can just not use them, or use with care.