Ice is what I use in warm weather. White hot, it glows black, black hot it glows white. I use a sandwich baggie, put enough water in it to make a 1/4" thick pack and freeze overnight.
I use 1/2" insulation foam board with a 1" square hole in the center, tape the ice pack to the back. Put a strip of duct tape over the hole so you can see the POI. It is hard to see the exact POI in the ice pack after you hit it the first time without the tape.
I can easily see the 1" square ice glow at 100 yards through my Trijicon.
