I use the D'Addario "between the strings" humidifiers and remove the soft sponge material and replace it with PVA sponge that can be cut and used as replacement.
The PVA sponge holds something like 10 times the water of the soft sponge and works very well. The built-in sponge pad in my Humidicase housing my Cordoba is this PVA material.
Here's a link to the PVA sponge block that I use:
https://www.amazon.com/Shurhold-210-...4050272&sr=8-4
If you're using water beads or some type of specially concocted process and you seldom need to add water then it may be a tip-off that it really isn't humidifying effectively.