The simplest humidifier is a sponge in a box. I've seen commercial ones that were simply plastic boxes with holes in the top and a sponge within. Wet it, shake out the excess, and put it in the guitar case where it can share the atmosphere with the guitar. It's better in the main part of the case instead of the picks & capo & strings compartment, but even that would help. You can probably make one if you can find a plastic box of some sort, and there are so many things that come in plastic these days you should be able to find something.

Be sure to shake out the excess, though. A leaky sponge in the case may lead to mold, if the interior plush lining gets wet.

The fancier humidifiers are sponge-filled plastic tubes of one sort or another that go into the guitar (with a clip so's they don't fall all the way in). If you have some old pieces of hose (garden hoses are rather large, but would work -- medical tubing is perfect), you can try stuffing thin-sliced sponge into one and sealing the ends. Don't forget the holes in the tube! Air has to circulate, to get the water into the case's atmosphere.

Bob