This is a box to hold silica gel desiccant beads for filament spools stored in 4L cereal boxes. There are a number of other designs out there but I designed this after spilling the beads too many times. My design requirements:

• Spillproof. Unlike other designs that have lids or other non-secure locking mechanisms, I wanted a box that would survive being turned upside down or being dropped on the floor. This design is mostly a single piece with a small threaded cap. 
• No non-3D printed parts. I didn't want to rely on screws to hold the box together.
• Maximize the space in the box. The cereal boxes aren't that space efficient so let's use as much of the empty space in there as possible and fill it with desiccant. This box is designed to curve around spools.

You can either use a single box at the bottom or, optionally, if you'd like your filament to be wrapped in desiccant, a second box fits on top (at least on my cereal boxes). I get to <5% humidity with a single box. A second box will reduce the time between needing to recharge the beads.

The cap threads are very fine. To close, align the cap's arrow to point up and turn clockwise ¾ of a turn to point left. Do not overtighten the cap.

I use Aqara humidity sensors and have included an optional box mount. Mounts for other hygrometers could be designed to press fit into the holes in the box.

I printed these in clear PC so I can both see the color of the beads and recharge the beads directly in the box.