I'd use either a heating cable or small heating pad designed for reptiles in combination with a dimmer switch that you can buy at home depot. Start low and work up to where you want to be.
These are in a formicarium with a water tank underneath it. I actually already told him not to put the formicarium on top of any heat source. But putting one of those things inside a small cabinet along with the formicarium would work if the specs are right.
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Senile Santa Like I was telling you, a 60 watt bulb shining onto the out world would be fine. Just make sure it's not too close.
I wouldn't be too worried about trying this, because It would probably have to be pretty close to heat it up enough to damage the fungus in the chamber below the out world. At that distance, it would probably be hot enough to cook any ants under the light as well. Even in that situation, the ants are at least "smart" enough to not stand directly under a light that hot, and will move to a cooler location.