Four (5) ways to mount a hygrometer/temperature sensor onto a filament spool. I bought the meters from eBay to monitor the humidity of my spools in storage.

The first is just a connected pair of cylinders, one to fit into the spool hole and the other to put the meter into. Useful where the spool hole is smaller than the meter.

The second is a 'claw' version of the same, to provide a little springiness but mainly because some of my spools have an 'outer' hole with spokes to the inner hole.

The third is for when your meter can fit into the spool hole (e.g. MasterSpool), and you just need a front panel so it doesn't slip inside.

The fourth fits the meter onto the edge of the spool so you can stack the spools sideways. It worked well for the first spool I measured. Then I found I have spools 53mm to 65mm wide. I've yet to work out how to make an edge mount that'll work for this range.

(V2) The fifth way is my final one. It's edge-mounted, but one edge only so it copes with spools of varying width. From this, I've discovered that even the edge differs between spools, and it easily falls off some, but this one's close enough for use in bags. Print it with the outside on top (unless your removed-support side is also presentable).

All but the first two will allow the spools to be stacked, as they don't change the size much.

Inspired by https://www.thingiverse.com/thing:2429114