I have been using hand warmers to sight in my thermals, which, works sufficiently.

Watt are other options that may work as good or better? I read here that aluminum duct tape was a good alternative. Anyone had good fortune with other things?

Lazy, I am. I place a square of Reynolds Al foil on the target, making sure the sun is shining on it. Even a dark black or red sticker works, again making sure the sun is shining on it.

Which means I don't try to sight-in on overcast days.

I always try to do final zero under same conditions as i hunt--dark and cool.
I have tried several things but prefer square/oblong hand warmer with a strip of black gorilla tape on each side except the top half inch.
This allows me to aim for top left or top right corner, speeding the whole process for my thinking anyway.
Also cleaning the gun often saves ammo in the process.

I use bright orange target circles & they look totally black & fuzzy. Am i I doing something wrong?

I cannot do that. I am relegated to a sunup/sundown city range.

Haven't tried orange, prefer foil or black (marker or gorilla tape strip). As for the fuzzy aspect, could be a focus adjustment?

For a thermal?

Silver HVAC tape works good. Hand warmers didn’t work for me when I sighted my thermal.

https://www.homedepot.com/p/Nashua-...-Foil-Sealer-Duct-Tape-1906075/100030120

What does your sight picture look like? Mine looks like a camera lens fading to black with a dim white fuzzy patch from the hand warmers. It worked but wasn't what I expected with my first experience with a thermal.

You can use a washer of whatever size you prefer. I just pin it on cardboard and heat it up with a mini torch lighter. It'll hold the heat plenty long to get back and shoot. I usually shoot a washer at 100 then handwarmers at 2 and 300

Several options based on sun conditions.

Easiest with strong sunlight is a black sticker/dot on a white sheet of paper. Sun warms the black more than the white. This is my favorite if there is strong sunlight on the target face.

Foil, either tape or just foil, on a black background. Works the opposite as the foil stays colder than the black in the direct sunlight.

If over cast then an AA battery. Roll in a few layers of foil and then close the ends causing the battery to short and warm up. Cut an X in the target and insert the battery on end. Gives a 1/2 inch diameter hot spot for about 20 minutes. The battery only lasts for one impact.

I prefer Duct brand chrome duct tape from Wally World cut to square sizes of choice with scissors spritzed with silicon spray for clean cuts.

Far easier than using heating foil tape.

Tilt your target frame a few degrees back for a better reflection from the sky.

And you can easily cut little squares to repair the holes.

https://www.duckbrand.com/products/duck-tape/colors/chrome-188-in-x-10-yd