I hope this isn't too off topic, but I have doubts about case humidifiers in general. I was really worried about this a while back, because I live in Phoenix and I was concerned I would desiccate my nice guitar and cause cracking. I learned quite a lot on the topic, but I'm not an engineer, luthier, or even a well respected forum member with an opinion likely to carry weight, so I'll avoid going into detail.
I concluded that offering my guitar an environment of stable temperature and humidity over the long arc of time was more important than offering it a humidity source like watering a plant. The exact value of that stable point was less important than hitting that point gently, but assuming that the construction of the guitar was probably done using stable wood at a mid-range humidity, the most gentle result would be a mid range humidity for storage as well. Low percent changes and slow equilibration was my goal.
It turns out my worries were probably unfounded. It's likely that I have an easier time of achieving storage stability than many of you who have already posted above me. I see Maryland, Massachusetts, Minnesota, New York, and BC - all places that freeze in winter and are lush green and humid in summer compared to my area. My house indoor temp varies from 65 in cold weather to 75 in hot weather and the indoor humidity near where my guitars live appears to vary between 30 and 50%. The humidity appears to be more affected by indoor temperature than time of year.
I decided to further stabilize for the guitars, though, and built this:
It was a Goodwill find that got pretty heavily reworked as a DIY project to make a sealed guitar cabinet. For me, the outcome is ideal. I was planning to add a humidity stabilizer using about a liter of saturated potassium carbonate in a glass jar with a Gore-Tex lid. In theory, that should equilibrate to about 43% RH. However, I put a hygrometer in the cabinet and found the RH was already at 55% so I just decided to watch. It hasn't varied more than a few points up or down ever since. I have 2 kg of sealed K2CO3 waiting to make a stabilizer jar, as plan B
Granted - this is not a workable solution for many people based on space, cost, time, DIY interest, etc., but I put it here as an example of what I believe to be a definitive humidity solution for a fraction of the cost of a commercial guitar cabinet. I probably put about $300 into this over a couple of months.
"If I have seen further it is by standing on the shoulders of giants." -Sir Isaac Newton
Armin Hanika 56PF