Clean and simple magnetic mount for the commonly used mini hygrometers, available on amazon and other places. They have a diameter of ~42mm (45mm on the outer bezel). 

The model is designed to fit two 8x2mm magnets, and the parametric Fusion 360 model file is included if you want to modify it for other sizes. The magnets are vertically aligned, allowing you to mount it to poles and metal shelving like IKEA Omar. I used a drop of CA glue to secure them. 

Printed with 0.4 nozzle and 0.2 layer height. Using concentric infill for top and bottom layers. When slicing, make sure that the thinest part around the notches is at least two layers wide if you want a good result. I used seam painting in the slicer to put the seams on the underside of the model. 

The hygrometer mounts with a loose friction fit. If you have trouble with fitment, you can adjust the parametric Fusion 360 model as needed.