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A good homemade case humidifier can be made using wet floral foam and a used pill bottle. The floral foam is the stuff you’ve seen used in flower arrangements and can be purchased in the hobby area Walmart, etc. I’d bought a couple of sound hole humidifiers and noticed they used this same material. There is also dry floral foam. That foam will not absorb water.
Just drill holes around the sides, top, and bottom of the pill bottle. Then push the bottle top onto the foam just enough to leave a mark. A knife can then be used to carve out a piece of foam that will fit in the bottle.
If you have one of those sound hole humidifiers, you’ll see there’s not much foam material. The pill bottle humidifier will hold much more water and has a larger surface area, so it will release moisture far longer.