A hygrometer measures the amount of humidity in the air, so why not a water droplet as a case for it?

The hygrometer used as base has approximately (LxWxH) 25x45x15mm.

For the magnets I used 2 of 10mm diameter and 2mm thick. It was enough to place on the fridge and even hold a sheet of paper. The hole has the same height as the body, so if you have a thinner or thicker magnets, they will fit. No clue was required to attach them, just the friction fit was enough.