After watching bigclive's recent video on a freaky dehumidifier I e-mailed the manufacturer to ask if it could work for a gun safe and they said "absolutely, if you have power available inside your safe."
They gave me pricing ($166USD plus $12USD shipping) and information (draws 3 watts continous) on the model RDH-31P https://www.micro-dehumidifier.com/p...ucts-with-psu/ which is good for up to 12cu ft (Stack On 18-Gun Cabinet from Canadian Tire is 12cu ft). For larger safes (up to 35cu ft) you would need RDHC-5P1 which is $344USD plus $40USD shipping.
You would need to drill a 1 inch hole through your safe then mount this device inside your safe, over the hole you drilled.
Unit we're talking about is on the right:
What it looks like inside:
This is the back of the device that mounts against the safe wall/over the hole you drilled:
I think I might give it a try
-Jamie M.