Do you even
need a dehumidifier?
A closed door to a safe, even a "breezy" stack-on safe, is going to prevent most of the circulated air coming in from the rest of the room. While not completely air tight, the closed door will do a decent job of preventing that ambient humidity from entering. A medium-sized desiccant pack will dehumidify a stack-on safe quite well.
But like I asked before...do you actually have a humidity problem in your safe? General guidelines: "Iron and steel will not rust if the air in contact with the surface has a relative humidity below 50% relative humidity (RH)."
According to the NRA:
50% humidity at 70 degrees is considered ideal "Arms Museum" storage conditions. Any dryer then that and you could potentially have problems with wood stocks drying out and shrinking / checking over a long period of time.
Buy yourself a good hygrometer and measure the humidity where your safe is located...and measure it a varying times of the day. That will tell you if you really need to be concerned with a dehumidifier / desiccant at all.
In my experience, rust prevention has more to do with regularly wiping down your guns to remove hand oils than it does with humidity control.
That said, I use desiccant because I live near to the ocean. I use "DampRid" which is cheap and readily available at most grocery and hardware stores. It comes in a variety of container sizes and shapes. It works great, but does need to be replenished every so often with new powder.
An alternative is to put a 4 or 7watt incandescent nightlight inside your safe. Works exactly the same way as the grossly overpriced "Dri-Rod" dehumidifier. Provided your safe isn't the size of a walk-in closet, the heat generated from the bulb is just enough to circulate the air and slightly raise the ambient temperature inside your safe, which in turn lowers the relative humidity inside the safe.
I hope this helps....good luck!