Substrate thickness won't make a difference in controlling the temperature of the heat mat, because the snake will just move it all out of the way and sit on the glass anyways. Your best bet to ensure you get the right temperatures is to get a thermostat or rheostat and attach it to the heat mat (because all heat mats get FAR too hot for a ball python), and if the room is too cool then invest in a little space heater that you can get for dirt cheap to keep the temperature between 78-80 F.