I get cold feet when I winter camp. I spend most of the night wiggling my toes and rubbing my feet together, so I don't get much sleep (the rest of me is comfy, I've got my Icebreakers on). I've tried synthetic socks, wool and silk. The only thing I've found that really works is putting one of those chemical hand-warmer packets on my heel, between the liner and outer sock. But that's impractical.

What I need is a down-filled, sock-type thing. Something that will leave me writhing in infernal agony over how hot my feet are. When I web-search it I get "camp booties", but I've tried those; they're too loose and not very warm. Does what I want exist, and what is it called?

tl;dr: Where can I find mini sleeping bags for my feet?