I have a horned frog in an 18x18x18 vivarium.the temperature is around 80f and the heat is maintained by a heat pad that is on constantly and sits under the viv.I also have a thermostaticly controlled ceramic heat lamp.I was informed at first to place the heat pad on the side! this helps to heat the room more than the vivarium as radiant heat convects upwards not side ways.The side mounted heat mat would require an insulation backing to get as much heat as possible through the glass.I have positioned my heat mat so that it is underneath half of the water dish and half of the substrate.The frog finds its own comfort zone and the heat radiates upwards.Place a thermometer low down touching the substrate as this is the temperature your frog is experiencing.
regards
keith