My one guitar is warped and the frets thrusting out the side of the neck. It was stored in the original cardboard box for the first fifteen or so years, went into a soft bag about ten years ago, and came out (to be sold) when I "replaced" it with a secondhand acoustic-electric Nylon. It never had any problems, seems the last two years was the killer. All my guitars are stored in soft bags, more convenient than cases, space and moving around -wise. It has been a while since that guitar received strings and fingerboard maintenance (which I only learned of a few years ago, and doing Lemon Oil then), so part of it is my fault.
Humidity can be controlled in small areas with DIY solutions, making up your own humidifier/controller gadget is not difficult, once you have a suitable container and the suitable chemicals. The humidity controller is filled with a salt, all salts have an intrinsic relative humidity once saturated - they will release moisture or absorb it to try to keep that relative humidity. Of course, you will have to damp the salt when it gets dry, but living in the Congo you would probably have to dry it out in the oven every week or so... Once enclosed in a container, the "humidifier" will keep the inside at that specific relative humidity.
I still have to see how I shall do this with the soft bags I use, the long-term plan is (was) to build a cabinet with all my guitar stuff inside, which will then imply I humidify the inside of the cabinet What I think, is that keeping the acoustics zipped up in the bags helps. Well, slows the moisture loss/gain process through the years. I can do the humidifier-in-the-soundhole thing without a problem, but will that help with the neck too?
The offended guitar was for sale (I have not convinced any of the family/friends to buy it yet, and have not advertised it), but now I shall have to try and re-humidify it before I can sell it (at a lower price than previously, now "damaged"... It even has the original plastic protection cover on the pickguard! Ah well.). One way around this problem would be to only buy "cheap" guitars, play them for a few years (seems like ten years plus of "neglect" is possible) and sell them on when the top starts to wrinkle. All the time hydrating the fretboad as one should, to avoid neck problems.
I do have an old analog hygrometer, which I calibrated using table salt, but I do not trust it. It goes up and down, but I would not say it is precise enough for my purposes. Time to invest in a digital toy.
You will have to shift some of the columns around here, but the basic info is there. Seems one should go for Magnesium Nitrate. "Johnny calls the chemist.." Yes, seeing as we all are on good terms with a pharmacist, what with sore wrists and fingers and gout and so on, it should be possible for him/her to obtain what you need.
A saturated salt solution is made up as a slushy mixture with distilled water and a chemically pure salt. Or, just enough water to thoroughly wet the crystals, not runny wet.
Saturated Salt Solution Temperature (ºC) (deg F)
0 5 10 15 20 25 30 35 40 50 60
Relative humidity (%)
Ammonium nitrate 75 70 67 64 60 53
Ammonium sulphate 82 82 82 82 81 81 81 80 80 79
Magnesium chloride 34 34 33 33 33 33 32 32 32 31 29
Magnesium nitrate 60 59 57 56 54 53 51 50 48 45
Lithium chloride 11 11 11 11 11 11 11 11 11 11 11
Potassium sulphate 99 98 98 98 98 97 97 97 96 96
Potassium nitrate 96 96 96 95 95 94 92 91 89 85
Potassium chloride 89 88 87 86 85 84 84 83 82 81 80
Potassium acetate 23 23 23 23 22
Potassium hydroxide 14 12 11 9 8 7 7 6 6 5
Sodium chloride 76 76 76 76 75 75 75 75 75 74 75
Sodium nitrite 65 64 63 62 61
Sodium dichromate 61 59 57 57 55 54 53 51 50 49 47