D20 Countdown Dice Holder (Screw top)
thingiverse
The original countdown die by kittka is a standout - I initially downsized it and remixed it for various magnets (not shared, as I relied on an unsightly mix of magnets I had handy). A friend's puppy got its teeth into that, so I was more cautious about magnets for the replacement. Update: 2019/9/21 I've added a remix under "D20 Dice Holder (Screw top, non-countdown)" - https://www.thingiverse.com/thing:3871911 for those folks looking for an RPG d20, as opposed to a countdown d20.
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