I gave up
Louis' I used a little wild cat {approx} propane heater for a couple of winters with a inch or so of the hatch open for air. It warmed up the air maybe 20 d. Problem here is its usually 22 Dec and Jan, ice usually around the edges of the lake. I put the coffee pot on ready to light in the AM, sleep in 2 bags with long johns, couple of sweat panse ans sweat shirts and [very important] a stocking hat over the forehead and ears. I sleep warm and comfy and boy is that hot coffee good in the morn. Then as the sun comes up, I start delayering as the days get up to about 50. If the wind gets up I wear a vinyl type coat with a hood over the stocking hat and gloves--really good, the vinyl stops the wind from cutting thru. PS Please do not tell anyone that I do this.