The indoor range that I belong to doesn't like shooting the handwarmers. A good alternative is to fill a balloon to the size you like with hot tap water or just freeze it. Either way provides a differential in heat and a great target. When frozen you get to keep shooting a hit target as the ice mostly stays inside the balloon but the water is gone on first hit.

Done the frozen water bottle thing..

My buddy runs this shop and I have used his targets..

https://www.downrangethermal.com/product-category/thermal-targets/

If your thermal scope will see foil, foil works great. We use a 1.5" washer hanging on baling wire in front of the target often. Heat up the washer with a propane torch, and it's good for 10 minutes.

They don't like hand warmers but water and ice is OK?

Foil worked great for me. It’s probably overkill, but I actually taped mine such that the face of the foil was angled toward the sky. There was no mistaking the heat differential between the paper backer and the foil reflecting the cold sky.

I am new to R scopes, but I found that for outdoors, if it is a sunny day, a white sheet of paper with black electrical tape in the form of a cross works great.

The sun warms the cross up and it is green/yellow with the Pulsar Core and the white paper is black.

With the tape cross, it is easy to see how much vertical and/or horizontal adjustment is required.

That is with the ambient in the 50 to 60 range.

I use aluminum tape.

Foil works great for me. Easy to cut into a small square and tape to the target.

Planning on picking up some aluminum tape and trying that out. I heard it works great and is cost effective.