While I know this adds complexity to the matter, I used a very small and inexpensive ultrasonic humidifier to maintain humidity in the incubator.
At first I was using sponges and trays but I wasn't satisfied with the amount of fussing around I had to do to maintain proper humidity. I bought the humidifier and set it next to the incubator (a modified Styrofoam cooler) and jammed a piece of garden hose between the discharge of the humidifier and the cooler. I varied the alignment of the hose with the hole to vary the amount of moisture introduced. I used distilled water to prevent disease and deposits in the humidifier. This worked but was messy - I had water running all over the side of the incubator and had to fill the humidifier daily.
Next I disassembled the humidifier and cut one of the wires to the float switch. I extended it to the outside and reassembled the unit. I then hooked that up with a relay in series with the switch and controlled it with a Raspberry Pi and a DHT-22 temperature/humidity sensor and a simple Python program (I say "simple" but it took me a LONG time to learn enough to write it!!) I have two humidity levels set in the program selected by an external switch - one for the incubation period and another for lockdown. The relay turns the humidifier on and off on a 10 second cycle, varying how many seconds it stays on in response to the reading from the sensor to vary the amount of moisture introduced and maintain the correct humidity level without wasting any water.
Temperature control is currently accomplished using an industrial temperature controller and a 75 watt ceramic infrared heater (commonly used for reptile terrariums).
I know I probably have a lot invested in this for what it is, but it's a hobby for me and I have learned a tremendous amount about the Raspberry Pi and how versatile it is. Hopefully one day my life will calm down a bit and I can finalize the project - using the Raspberry Pi exclusively, without the external temperature controller, and adding egg turning, camera, charts and alarms and other bells and whistles. But for now, maybe my humidity solution will be helpful to you.