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The Golden rod has been the safe dehumidifier over here for decades.
So I guess they work.

I dont have a safe, I have a portion of the house.
I use a room dehumidifier here in the basement, when the floor gets damp and the sump pump is running a lot.
Other then that I use nothing but 5 w 30 syn motor oil on all my guns for all lubing and protection needs.

We bought a Fort Knox safe box ..has the hole already in it for powering dehumidifier and lights - if we fit them..

Depends on your level of "competence" in electrical wiring and ensuring teh safety of it.

All the heaters are is a piece of resistive wire, probably in a sand insulator with resin to hold it stable. At £50 each they are expensive and will need a transformer too.

You could choose a 50W (rated) 4k7 resistor and feed it from 240v ... Here

Or a 50W 12R resistor and feed it from a simple 12v "wall-wart" transformer used to power electronics - provided it is capable of 1A. You must have an old one of these around - check the output voltage and calculate the resistance needed for 10-12W 12R resistor

Reading their description - a 12W will do 100cubic feet - a cabinet is probably 10 cubit feet so you can get away with even less ... If you want 4w then a 12v supply and 33R will achieve that.
It will cost you a couple of pounds rather than £50!