If you can find an old 35 mm plastic film canister, you can make a pretty decent case humidifier, especially if you have a rectangular case. All I did was take a penknife to gouge out a 1" cylindrical hole in the styrofoam interior, drill a few holes in the canister lid and added a moist sponge and voila! Make the hole just slightly smaller than the canister diameter and it won't move at all. (There's very little danger of this anyway, because when the top is closed, the lid keeps the canister from moving around the case)
Here's a photo. Enjoy!