You might just consider getting desiccant packs from McMaster Carr and see how those do.
Its been a while but I think they have ones that change color when saturated and you can cook out the moisture and use again.
You could buy a Hygrometer and put in the safe and check the level once in a while until you establish you are ok and then base the re-fresh on how the media is doing and trend is going. Establish a safe (pun) time period that is under the high limit you want and just change it based on that.
Your environment definitely calls for precautions.
You would have to drill a hole in the safe for the antennae though as no wireless is going to make it thorough steel.
Last edited by RC20; February 5, 2013 at 12:36 AM.