Metal coffee can with the lid. Take a roll of toilet paper and remove the cardboard tube, place into the can and then pour a bottle of Isopropyl or denatured alcohol and let the TP soak it all up then replace the lid. Once in the blind light it for practically odorless heat. Just make sure you have some ventilation and keep away from the blind material. I have used this many times in natural blinds and in tents to reduce that early morning chill. Very cheap and works really well with a little common sense.