Electric "rod type" dehumidifiers are used extensively by gun hobbyists. They are inexpensive, easily arranged in the safe, and effective in keeping moisture away. With no moving parts a rod will continue to work for many years.
Rods are best used near the floor, where the heat will rise to the top of the safe. Simply prop it upright against the front interior corner of the safe.
A heating rod requires no maintenance or watching of any kind. Unlike other methods such as silica gels, there is no "down time" when the product does not work.