I do not use damp rid in much anymore. I have never tried their products in the proposed location. Most of their products are calcium chloride based. As the product absorbs moisture, it turns into a slurry of saltwater. The regular stuff may be useful in a gun safe, but it produces a liquid you dont want to have come in contact anything metal. They do have a charcoal based product but I believe it also has the regular stuff in it as well. You dont want to have any of the hanging bag units laying down, as the liquid produced can leak out.
If you arent already using something that is similar, I would suggest searching for Hydrosorbent Silica Gel Dessicant Dehumidifiers at Midway or another retailer. I use them in several places and can say they work great.
http://www.midwayusa.com/product/799...-feet-canister