The mountain men use to bury coals or heated rocks about 6" deep or so to stay warm on cold nights.
Dont think I would try coals with synthetics though. Better use a wool blanket until you get the technique figured out.
Not sure how you would do that night after night with a small debris hut either.
Probably better to use a lean to for that one and just keep a fire out front also.
There is a youtube where Dave Canterbury has a tiny fire in a hole (not a dakota) with a flat rock over it in one of his debris huts. He also talks about building a small fire inside to smoke out all the spiders and whatnot which makes sense.