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I use this heater: https://www.amazon.com/dp/B00MXEDAV2/?tag=exoticpetnetw-20
Make sure that you make some sort of cage or shield to protect the tortoise from coming into contact with it. I'd recommend putting it on a dedicated thermostat also.
Are you able to raise the ambient temperature in the room (especially at ground level) up a little? Ideally the minimum ambient temperature for sulcatas is around 80F.
I use a oil filled radiator connected to a thermostat to regulate the temperature indoors for our sulcata.
Where are you located? What's the room temperature indoors where he's staying? What kind of heat are you using?
I'm sure others will have advice also but that information will help give you the best advice.