The OEM caps for Essick humidifiers are absolute garbage. We go through a set every year because they crack and fall apart. I got fed up with buying new caps because they made junk. This is a beefy version that uses the internal mechanism from the OEM cap, but should hold up for much longer.
Designed around a nut generated from this model: https://www.thingiverse.com/thing:193647
Must be printed with supports.
Remove the rubber diaphram, stem, spring and o-ring from the OEM cap. Install in the same manner on the new cap.