Finding myself with a few dozen small round RH/Temp meters (cost $1-$2 each) I started adding them to my filament bags. By doing so I'd have at some idea as to the actual storage humidity.
They'd bounce around in the arbor (the spool's hollow as it were) making reading them a bit of a chore - and not the best for the bags either.

So
I created these disks into which the meters snap right into place,
The disks have a shallow sleeve which slips into the spool's arbor (the round open) and serves to keep the disk in place. The sleeves are shallow enough to no interfere with battery changes.
I have included four variations. They vary by the diameter of the sleeve which extends into the spool's arbor.

Included models - stl and 3mf

52mm - Humidity Sensor Spool Mount 52mm
               52mm fits most spools allowing a few mm of play

70mm - Humidity Sensor Spool Mount 70mm
               for those spools with a large arbor, e.g. SunLu Matte PLA. 

50mm Keyed -Humidity Sensor Spool Mount Keyed 50mm
                  Some Prusament spools, e.g. PC-Blend, have a notch which extends from the 
               spool arbor (arbor) further out into the flange.
               They also have a smaller arbor. This model has a 50mm sleeve along with a small
               extension which fits into the notch, a key as it were.
               -see image of black spool with red object-

50mm -  Humidity Sensor Spool Mount 50mm
                Same as 50mm Keyed, but without the addition of the key. 
                Some Prusament spools

NOTE:   size, 52mm, 70mm, 50mm; refers to the outer diameter of the sleeve insert. 
               The diameter of the opening for the RH Meter is 41.3mm 

Printing 

• PLA, PETG …   (in attached photos, black and red are PLA, gray is PC Blend)
• No supports.
• 15% infill - type is not a big deal, I used Cubic for what it's worth
• Top/Bottom layers 3 or more
• 0.20 layer height - sleeve slightly tuned to assume 0.42 layer width