I have two damp sponges loaded in the compartment of my molded Martin case that seems to hold humidity well. So well, in fact, that the first year I had that guitar, I overdid it a bit and had to bring the top of my guitar back down as it "bellied out" a bit. I was really bummed, but it came back into a correct shape without cracking with careful backing off on the humidity. The thing is to avoid big fluctuations. So be careful with your case humidity! Get a cigar humidity meter thing and stick it in there too to keep track.