A friend of mine wanted a Zyn case that was:

1.) As small as possible

2.) Holds a full pack of 15 *(EDIT: Thanks to the suggestion by @hogohogo, I have also added “Euro_20” versions for those in countries that sell 20-pouch packs)*

3.) Is not obviously a Zyn case

4.) Can be quickly opened with one hand

This is what I came up with!

The lid rides along a pair of rails which - depending on your print settings/filament - may need to be worked back and forth for a minute to achieve the desired smoothness. The lid stays shut using eight 4mm x 2mm magnets (4 in the base and 4 in the lid). I modeled the magnet holes aiming for a press-fit, however since the tolerance required for these small magnets is tight, you may need to use a small dab of superglue underneath each one. It is likely obvious, but make sure you orient the magnets such that the lid is attracted to the base, and all magnets in each part are the same orientation (aka make all 4 in the base attract to the same polarity).

NOTE: I have also included the models for an “extended” version that is slightly larger, the primary model is just barely large enough to fit exactly 15 (or 20 if using the “Euro_20” version) pouches, if you don't mind an extra 0.25" length, the extended version makes it much easier to fit the last few pouches when refilling. 

TLDR: Normal versions are exactly the size needed to fit a pack, “Extended” versions are a bit larger and therefore easier to refill.