I'm not sure if these next questions are silly or not, I'm just nervous about doing anything wrong as it's my first time trying to make one of these set ups!

So you need a slight gap between the tub and heat mat to allow air circulation but I'm wondering about what you put the heatmat on, like the underside of it. I'm going to put my tub onto a wooden dresser, it wouldn't burn it would it? Also how big should the gap between rub and mat be?