Two approaches come to mind if you can't control humidity in an entire room.
First is heat. Raising the temperature a few degrees in a closed (but not airtight) container will lower the humidity inside the container. Best devise for this that I'm aware of is a "Goldenrod" or equivalent. But a low wattage incandescent light will also work.
Second is a renewable desiccant such a silica gel. That's what is typically put in the tiny bags you see packed with new photo equipment. It can be renewed by putting in an oven at low temp for a few hours. Some forms have a color indicator to let you know when it's absorbed humidity up to it's capacity. I have a few 1 pound bags that were originally shipped with imported motorcycle engines. This might be your best bet.