This is a mount for the popular circular hygrometers (humidity sensors) to fit within a Prusament spool. It's held in place in the spool by friction, and the hygrometer clips into the mount.
There are several other posted models for mounting a hygrometer in a Prusament spool, but the ones I saw all included a desiccant container right behind the sensor, which I felt would not do a good job of reflecting the ambient humidity in the space. See https://www.prusaprinters.org/prints/36207 or https://www.prusaprinters.org/prints/35562 for examples if you prefer that approach.
The specific hygrometers I purchased are https://www.amazon.com/gp/product/B07GR67G1M/ and measure ~41mm across the back. The mounting hole is 41.5mm wide to accomodate the built-in clips, so it holds steady but can be turned by hand within the mount. You'll need to resize the mounting hole if your hygrometer is a different size or doesn't have the same type of clips.
This prints easily face-down, I printed it with PLA at .20mm with PrusaSlicer's default settings.
EDIT: I re-made this model in OpenSCAD and uploaded the file, so you can now customize the opening
or any other variables as needed. The original model was made in TinkerCAD.
The author marked this model as their own original creation.