I used an old wool blanket in my 1 man clam. I didnt to it becasue of light penetratikon but for warmth. I out the spots that I had to for windows, vents, and poles ect. Than just slid it up inside of the canvas. Its dark as heck and a heater is just about not needed. I also left the corner that i put my heater in void of the blacnket due to fire hazards
Once in a while it will get alittle wet but it just slides right out to be dried. Its a cheap easy fix. Fleet darm has the old army surplus wool blankets for 10 bucks , I think? or check your local st vincent de paul or goodwill. Good luck!