A lot of great ideas on this post. Many sound like a lot of extra effort. All I've ever had to use was tin foil folded to about a 1 1/2 piece. Pretty simple and trouble free. It always works cold or hot day or night. Sometimes you get a better image with black hot or switch back and forth to find the right combo with each different situation.
Done it 100's of times. Check my zero often as I change between day and night optics often on two of my guns.
It may also make a difference with the quality of thermal you have? Mine are all trijicon.
