OK, here are some more options for DIY case and soundhole humidifiers.
We've already discussed the gel beads which can be found in the floral aisle at Walmart, Joanns, Michael, Hobby Lobby, etcetera.
For holding them, you might also consider "
organzabags". They are mesh fabric bags which are very easy to use and seem pretty secure. They also are very open, allowing the beads to breathe. Use caution however, since direct contact will cause wetting of the surface, so you don't want these laying inside your guitar touching the wood. They won't drip, but if they are in direct contact there will be wetting.
If you are looking for something larger, an ideal material is
cross-stitch "fabric". You know this stuff: little old ladies make little yarn pictures or coasters or whatnot with this stuff. Its basically huge white plastic window screen. The nice thing about this stuff is that mesh openings are the perfect size: they are very wide but small enough that the dried up beads do not fall through them. You can cut a piece of this and hot-glued it as a window screen on a soap dish or something.