I think you are definantly on the right track. Perfect location too. Had a similar one in a previous house I had. Just to keep noses out, and safe from the kids.
I actually had my rifles raised off the floor by a couple feet on a shelf. Good for storing ammo under, since ammom is heavy. I had mine so I could stack two ammo cans and have plenty of room to spare.
Maybe a can or bags of desiccant in there. Rice, or at a craft store they sell silica.