Holder for the cheap little thermometer/hygrometer available on Amazon (part number B07BLY177S). Along the same lines as my ugly little holder but now with magnets for mounting to steel surfaces.
Uses 8x3mm magnets which are also available from Amazon (part number B07B3SDRCP). I have many of those on hand since I bought them for some other project and it comes in a pack of 100.
Print out the thing, and then press a magnet into each of the holes. For mine, I found sticking the magnet on the end of a 6mm hex T-wrench helped me push it in, as it was a nice tight fit. Press the magnet so it's flush with the back edge for the best holding power.