There are a lot of things you can use. It doesn't have to be hot, you just need a temp gradient. In the past I have used: water bottles from the ice chest, ice cubes in a zip lock bag, foil tape, hand/foot warmer packs, frozen pancakes, frozen grapes, and I'm sure a few other things I can't remember at the moment...
Keep in mind, the larger the gradient, the more heat radiation you will get which makes the target look larger than it is. Turning your brightness and contrast down will help a bit.
What I use now are 1/4" fender washers. They are about 1 1/8" outer diameter. I just put a screw or nail into the backstop and hang them. I do not heat or cool them first. They will absorb heat at a different rate than the backstop material which gives enough heat gradient.