DampRid / Calcium Chloride
They have packs made for putting in clothes hampers, tackle boxes, closets, etc. Is this stuff GTG for putting a package in a safe to absorb moisture?
You absolutely, positively must not allow CaCl2 to touch metal.
Make sure it's in a plastic bowl or something like that where it won't spill or contact anything in the safe or it's walls..
It's highly hygroscopic and the chloride will rust your shit on contact faster than you can blink.
It makes a great dessicant absent the corrosion issues.