Originally Posted by PINEKNOT
Silver HVAC tape works really well to sight thermal in the daylight, even on a bright sunny day. I tried hand warmers, but had trouble seeing them.

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

I had good luck with an "X" made with black electrical tape on white paper.