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.