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This is a 6L dry box roller insert designed to work with this or any similar cereal box:

https://www.amazon.com/gp/product/B081J8CY7B

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

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

The rollers use skate board bearings (aka “608 bearings”) as the pivots. These are the ones used in the photos:

https://www.amazon.com/gp/product/B07FF1RBPV

The spigot insert for the PTFE tubing is designed to use the 9mm stem black/silver fittings from this assortment:

https://www.amazon.com/dp/B07PV8754Z

Note that that's not actually a 9mm stem, it's a 1/8" NPT pipe thread. The center hole in the spigot insert is the nominal tap size for a 1/8" NPT tap. The hole in the model itself is not tapped. You will need a 1/8" NPT pipe thread tap in order to complete the project (be sure to select the 1/8" NPT size when ordering!):

https://www.amazon.com/Drill-America-POU1-8NPTW-DRILL/dp/B01M4RXY21

DESIGN NOTES:

Aside from the rollers themselves, the only other 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

We designed the roller insert to make it so you don't have to remove any in-hub dessicators. This makes swapping spools easy and makes it so you're not having to keep track of yet another dessicator full of color-changing dessicant.

I wasn't happy with the fit of many of the other cereal box spool rollers out there and wanted something that fit like a glove. To that end, I made a urethane casting of the bottom of the 6L cereal box, scanned it on a flatbed scanner, brought that into CAD, and used that as the basis for my model. (See photos.) Tip for other designers: this is a really easy way to make sure your parts fit as closely as possible. The urethane I used is from SmoothOn. Nothing special about it for this application. It was leftovers from making hydrophones. NOTE: YOU DO NOT NEED THIS TO MAKE THESE PARTS:

https://www.amazon.com/gp/product/B00E3ZJ9XW

PRINTING NOTES:

Everything is designed to print without supports. The bezel for the hygrometer is made as a separate part so that you have the option to print it in a different color or material than the rest of the roller insert. It prints face-down so it'll take on whatever texture your plate provides. (I used the textured plate for mine and like the stippled appearance.)