What I would say is do whatever works for you and your animal. I had my mat on the side and I noticed my frog would sit ontop of the substrate at night, right on the glass... So I thought that if there was any chance of burning it would do the same with it on the side. Heat lamps would be the best option but for where I live I'd need a very high wattage to reach the temps needed and I found dried out the cage far too fast