When I lived up north I tried the aquarium heater trick with a water and he ended up breaking it. To this day I do not know why it did not fry the monitor in the pool but I can tell you that I got knocked on my ass when I touched that monitors head while he was in the pool. Needless to say I was very lucky all the way around.
I would suggest using some type of protection around an aquarium heater with waters. I would also suggest using some sort of conduit to protect the wires which monitors WILL inevitably get tangled in and cause problems. My second and last experience with aquarium heaters and monitors resulted in a nile I kept for many years resting on the heater in the pool and getting thermal burns. Both of these incidents happened early on in my monitor keeping days.
Your best bet would be to heat water outside the cage and circulate it into the cage with some type of pump system.
BTW they are called water monitors for a reason.

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