use a heat mat, your frog will move where he is comfortable it wont burn him as long as you keep a deep substrate like you should to start with. your substrate being heated helps the digestive process of your frog, using heat lights is a nono, you will just dry him out. you can also get a thermostat for the heat mat if your that concerned, but i dont use one on anyt of my tanks, choice is yours but if you keep a glass top on the tank with the heat mat you will achieve good humidity and good heating.