I recently got a Sunlu Filament Dryer Box and I’ve seen a few people mention that putting a hygrometer inside of it (and another on the outside) can give you a good idea of how much it lowers the humidity. Rather than just sticking it inside the top of the clear cover I decided to make things more complicated and put the hygrometer in the center of the spool so it could spin around and I could practice reading things upside down.
Note that every roll of filament from every manufacturer may have a different spool hole diameter. In face, even the same damn roll of filament may have different diameter holes on each side! Don’t bother printing this for a snug fit, just make it too small and add some tape either on the body of the print or on the lip to hold it to the spool. The spool is slowing spinning around so it’s not like huge forces will send it flying out into space.
You can get the STL and .scad file from Printables.com – Hygrometer Filament Roll Holder. Print it if you need it!