This is a remix of my original filament guide, and my filament drybox pass though. The primary improvement over the original is that this uses a 17.8 x 2.4mm o-ring around the ball, which should seal it better and also makes it a lot smoother. Scroll to the bottom for info on a new version of a spoolholder "system" I'm working on which will incorporate this thing and a few other things.

This was designed for a Rubbermaid 21 cup cereal box, , which I am using as a drybox to store filament. The wall thickness this is designed for is 2.8mm. The hole required is 34mm (1 3/8"). I used a forstner bit to cut the hole, but Sibicle commented that a harbor freight step bit worked well for cutting the hole. You should also be able to get the step bits cheaper than a good forster bit (and it will probably work better too):

I'd use one like this:
https://www.harborfreight.com/2-piece-titanium-nitride-coated-high-speed-steel-step-drills-96275.html

The Rubbermaid 21 cup cereal box is Amazon Item "B00XJRMW5M" (please copy and paste the item in the search field at Amazon, you can read why below)

This design, like the original it was remixed from, was inspired by (though not remixed from) YasiekPL's awesome "Filament dry box dispenser":
https://www.thingiverse.com/thing:3626260

This will require:

3x M3x10 screws and 3x M3 nuts.
1x P18 17.8 (ID) x 2.4mm O-ring (required for the ball)
1x 22 (ID) x 3mm O-ring or P22.4 22.1(ID) x 3.5mm (preferred size) O-ring for the inside of the cap - use the larger one if you can find it.

A short length of PTFE tube is optional, which is used to pass through the ball and provide a smoother filament path. It is not needed if using the remixed ball (options linked below), which use a pneumatic coupler.

I used an older version (91512) of the following O-ring kit from Harbor Freight which has the required O-rings:
https://www.harborfreight.com/397-piece-metric-o-ring-assortment-67580.html

I also used some clear silicone sealer on the inside and outside covers which should help to seal this further. I included a pic of my enclosure filament pass though which is similar, it shows where the P18 O-ring goes during assembly.

Make sure to check out the remixes of the ball by Madewithlinux's and Mandryd, they are modified to use a pneumatic coupler so a ptfe tube can be connected:

Madewithlinux's remix:
https://www.thingiverse.com/thing:3850124

Mandryd's remix using a PC4-M10-0.9 coupler (includes threads for the coupler):
https://www.thingiverse.com/thing:4035357

STEP file is included for easy remixing. If you make one, please post it, and if you have a suggestion please post a comment.

1/23/2021 - I have removed any Amazon links, and instead provided the amazon item numbers in quotes. This was done as a small protest against the way Thingiverse edited my description, and installed their Amazon Affiliate links without notifying users that they were doing it. Sorry for the extra step of having to copy and paste the item numbers, but I believe they need to be clear with content creators about this. I did not originally post my own affiliate links here - they were "clean" Amazon links, but for some content creators here, the affiliate links are the only way they receive anything back for their work, and that avenue is now closed off. More info here:
https://www.thingiverse.com/groups/thingiverse/forums/site-issues-and-feedback/topic:49539

Update 8/29/2021: I'm working on a new design (remixed from OcieWard's design https://www.thingiverse.com/thing:3138757). The remix will incorporate a spool holder, this filament guide, and some other things into a spool holder "system" for lack of a better term. The primary feature will be a base, which the Rubbermaid containers will latch into, and be easily removed by throwing a lever (quick release). So many words to describe it, when some pics will do the job: http://www.mystoopidstuff.com/blog/work-in-progress-spool-holder-for-a-rubbermaid-21-cup-container I'm updating progress there, but the design should be tested and ready in about a week or two (I printed the first prototype and it works pretty well, but I made some optimizations so have to print it all again). Update - here is the link: https://www.thingiverse.com/thing:4979880

If you find these models useful, please post a like or a comment with some pics of your prints.

I regularly update my current projects and other things I'm working on at my blog.
And if you like some of the stuff I design, you can follow me here or on Printables, to see what new stuff I post.

Thanks for looking!