Look up a Dakota Fire hole/pit. Also look up a self-feeding fire. One small one (about 6" wide) with that style of fire lay will toss a lot of heat for a long time.
Two, one at the foot end, and one at the head end, under your tarp will heat it up tremendously! Make sure that you use dry wood sticks, and no greenery (smoke). The hot coals will provide heat for a long, long, time.
Something to think about.
SkyPainter - "... and then the police came."
All these years people said,"He's acting like a kid!
He did not know that he could not fly, and so, he DID!
Live Purposefully; Dare Greatly; Land Gently
"Never knock on Death's door - just ring the bell and run! He hates that!"
My outdoorsy YouTube channel is: "WMICommissioner"