I use Eva Dry dessicant humidifiers which do not heat up the safe. Own 3 for 2 safes. Keep one plugged in and recharging and one in each safe. Rotate them in NC about once every three weeks in a very humid year here. Seem to soak up a lot of moisture. The beads in the viewing window turn pink when they are saturated.

http://www.eva-dry.com/products/eva-dry-500-mini-dehumidifier/