When I recently bought my new Traphagen guitar, Dake sent a D'Adario humidity system with it. I think it is actually made by Boveda, who makes a lot of like products. The system comes with two packs. One sits in your case where the headstock is and the other (which actually has two pockets) is supposed to loop over the middle two strings and hang into the sound hole. However, it is made for steel string guitars and is a little too large to fit well. Dake suggested just putting it in the compartment where you store strings, etc. There are a couple of advantages to this system. You just replace the packs that go into the pockets periodically instead of adding water, etc. The other one is that it both adds AND removes moisture from the air as needed, keeping the air inside the case at 50% constantly. when you have the guitar out of the case, you leave the case closed to maintain the humidity inside. If you have a wooden case, it takes some time to "season" the case. You can read about it on the Boveda site. They also have an hygrometer that you can calibrate. I have tested the system by placing an hygrometer in the closed case while practicing and checking it when I put the guitar up afterwards. The humidity in the case was 50% regardless of the humidity in the room. i realize this is not quite what you asked about, but it certainly helps you maintain a great environment for they guitar.
2011 Traphagen Cedar Doubletop
2017 Traphagen Spruce Doubletop