On the glass tanks, when sitting on a flat surface I added self stick feet like this https://www.lowes.com/pd/Scotch-Rubb...ers/1000328183 to the corners ,

that gave it an air gap for the heat. I also covered my heat mat with a piece of reflectix,

https://www.lowes.com/pd/Reflectix-R...5-ft-L/1014123

to help direct the heat up into the tank for a hot spot. I'm currently doing that on a plastic tub right now for my quarantined snake.