This is a holder for a humidity monitor (hygrometer). It is intended to be used when storing your filament in a vacuum bag, zip-lock bag, or a cereal container. The monitor assembly consists of 4 parts: the hygrometer, the holder, the side support and a rubber band (elastic band). Start by inserting the hygrometer into the holder and attach the rubber band to the loop on the back of the holder. Next insert the holder into the spool center hole. On the other side of the spool, loop the rubber band around the side holder so it is taut when the side holder is against the back of the spool. The humidity monitor fits most spools from large holes to small holes.

Notes:
I have used the following hygrometers:
https://www.amazon.com/gp/product/B0CMTNLK2H/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B0C74T4JH1/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Both fit nice and snug. One batch did have a slight burr on the side flange so it couldn't be inserted, but the burr was easily/quickly removed with a file.
If purchasing a different model make sure it can read down to 10% as some hygrometers only go to 20%. Also note that the hygrometers it is designed for show a diameter of 41.4mm. If your dimension differs the X and y dimensions can be scaled in the slicer.

In the pictures you may note that the humidity is fairly high - that's because I just put some older exposed filaments into the bags for the pictures. I need to dry them.