I went along to a reptile store and bought at stupid expense, (but don’t tell the wife) a “micro climate” thermostat and coupled it to a ceramic bulb heater. I chose the highest wattage he had, but can’t remember what it was! My reasoning was that it was better to have an over powerful heater controlled by the thermostat than an underpowered heater working its socks off and getting nowhere! In retrospect I wish I had either somehow got a more powerful ceramic bulb or installed 2, as I feel it does struggle a bit to get to the 20s.....but so far it does and has!