Hey Krankie,
I have spent the last 25 plus years opening and repairing safes. I have two very large gun safes at my house, and six smaller ones. The only thing I have ever used in my safes is cat litter. If you take a tuna fish sized can, clean and dry it really well, and fill it with cat litter, you shouldn't have any problems. I live in the Mid Atlantic area, just outside Washington DC where it gets very humid in the Sumer time. I have never had a moisture or rust problem with any of my guns or paper work I keep in my safes. If your safe is pretty good sized, put two containers of litter in there. Put some plastic wrap over the can and hold it in place with a rubber band. Poke some holes with a pencil in the plastic wrap, and forget about it for a few years. The last time I changed my litter was in 2002! When I got my big safe, it came with a Golden Rod. The thing is still in it's packaging sitting in a shelf. You can buy a lot of cat litter for the price of a golden rod. Get the kind with the little green crystals in it and it will keep things from smelling funky.
John