I made up a video showing how we zero the thermals prior to hunt. I use a Propane Torch to heat the 3/8" AR500 steel. It will stay visible in the thermal for about an hour with only a little heating.

I have seen where folks use disposable hand warmers, which will work ok, but I prefer the steel.

good stuff

your steel method is definitely better but most folks may not have
the luxury of the kind of backyard you have or ways to portage a 150lb steel plate
and have to rely on hand warmers

another method I came up with is to spray the paper target with cold water

Stupid question...how do you know where you hit on the hot steel? Walk down there?

Steel is painted white or other easily visible color. Hits are where the paint has been splattered.

Adjust accordingly. But, yes, it takes a drive down there on the UTV to check the hits and paint over them.

I just had a Hog silhouette cut out of armor steel so my guests can practice with what they will see in a field. It is 36" x 22". Heated up it will look just like a Hog with the thermal.

Another one will be on a cable system connected to an electric motor to simulate a runner from side to side.

One way is to get some mylar or some of that stuff a reflective party balloon is made of. It shows up cold in the thermal.

My targets are portable and mount to a Tpost. The 12x24 plate is about 45 lbs or so.
With a leveled tpost in the ground, it just slips over it and hangs there.

If you use a particular load for hunting, each and every time, then you can use a day optic to sight in at your optimal distance (200 yards for example) and then once you are zeroed shoot a three shot group at 25 yards.

Now put the thermal scope on and zero to the same POI at 25 yards and call it good.

For me it is easier to see targets at night regardless of technology (therm/digi/I^2) at 25 yards than 100 or beyond.

Cool idea on heating the steel. Handwarmers have worked well for me.

The margin for error at 25 yds is too obtuse imo for that to be what I would be confident fielding.
I zero at 100 yds and try headshots out past that distance when possible.

Also, the bore height distance between day and thermal optics most likely would be different further exacerbating the issue.

For 40 yd shots it might be fine for a minute of Hog zero.

I used a hand warmer taped directly over the center of the target at 50 and 100 yards. It worked well. It is a little more challenging to get it dialed in, but it worked. I know of a guy who used a torch to heat up a quarter and did the same thing.