I have these plastic boxes to store my filament roles, and to keep an eye on the humidity I bought some hygrometers online. 

They were not really meant to be mounted on the front of my boxes, so I created a simple holder I could attache using two screws. 

The thread size I used is M3.

The design was made in FreeCAD and I've included the CAD file as well if someone want to make any modifications.