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This is a 4L dry box hygrometer insert designed to work with this or any similar cereal box:

https://www.amazon.com/gp/product/B08JQ9P5L6

It is designed to take a round hygrometer similar to this one:

https://www.amazon.com/gp/product/B07FCPJRFH

The only feature of this insert is the hygrometer to let you know how dry your filament is. I didn't include any sort of dessicator. This was on purpose. Early on, we (R&T) found a really nice in-hub design for a filament dessicator that we've been using ever since (shown in photos):

https://makerworld.com/en/models/24256#profileId-21462

With the dessicator living in the hub of the spool, it's easy enough just to pull it out and stick it in your AMS or a roller dry box without having to remove the dessicator. Here's a link to the roller dry box insert we designed for 6L cereal boxes:

https://makerworld.com/en/models/241613

These inserts have slots to hold material tags (shown in the photos). I've included STLs of these for a number of filament materials, though the list isn't exhaustive. If you need a new tag, you can make your own (text is Arial Rounded MT Bold) or you can ask me to make an STL for the material of your choice and I can update the files.

When importing the material tags, import one of each material, right click it, select “Split” → “To Parts”. This will divide each tag into its component pieces. In the left menu, switch from Global to Objects. Each component of each tag will be listed individually. Select your text filament and make the entire tag that filament. Then pick the first and last part in each tag part list and change that filament to what you want your base color to be. Sorry if this is confusing. Play with it and you'll see which parts to change colors on to make the tags look the way you want them to. The ones in the photos were printed with white PLA Tough for the text and black PLA Tough for the tag and thumb handle.

DESIGN NOTES:

I wasn't happy with the fit of many of the other cereal box hygrometer inserts out there and wanted something that fit like a glove. To that end, I made a urethane casting of the bottom of the 4L cereal box, scanned it on a flatbed scanner, brought that into CAD, and used that as the basis for my model. The inserts are a nice, snug fit into the bottom of the handle portion of the cereal box. (You have to push them down into position with light pressure.) Once installed they have no tendency to rattle around at all.