My neighbor has beehives and they were talking about installing some kind of heater to keep the temp at 107F (42C) for a few hours or so when needed to kill bee mites.
Things to consider:
First, beehives are usually located in remote places, without mains in proximity.
Second, is it: "To keep the temperature 41.95C to 42.05C" or more like "40C to 44C will do"?
Third, how often would you like that to happen? You have to attend beehive anyway so is WiFi and autonomous operation necessary?
I like simple designs. I'd consider making one frame with a container with substance that changes state of fusion at a temperature little higher than 42C (experiment needed). Then you cannot f%@ck it up. No controllers, no wires, no electricity, no arduino and no bugs.
Ca(NO3)2*4H2O melts at 42.7C, is non-toxic and if you buy a pure one (and not the industrial or agricultural dirty grade) it will stay at precisely 42.7C all the time, till it completely solidifies. The state of fusion is around 120kJ/kg so perhaps first estimate how much energy your plan requires.