Simple mounting bracket for attaching a Xiaomi Mijia Thermo/Hygrometer (LYWSD03MMC) to the inside of a drybox. This is the same thermo/hygrometer bambulabs shows in the wiki page on drying filament. They're generally fairly accurate, easy to calibrate, and can be acquired for a few bucks each

Prints in two parts, a front and a back, with a shadow line between them. Easy enough to glue together, and fits snug enough you might not have to.

I'd recommend gluing the back to the holder, and then gluing the “window” side up against the wall of a drybox. That way you can see inside and measure the thermo/hygro inside your dry box, and can easily pop the unit in and out to change batteries. If you don't want to glue it to the side of your container, 3M VHB tape works very well, and is less likely to cause any visual effects on the plastic of the container.

This particular Thermo/Hygrometer can be flashed with a custom ZigBee firmware.

Once flashed, you can configure the little “face” on the screen to read as “happy” when your dry box is dry, and sad when its not. You'll want to set the following ZigBee attributes through your ZigBee platform (in HomeAssistant, you'll need to use the ZHA toolkit custom component).

All attributes are in Cluster 0x0204 (516)