Thingiverse denizens have been 3D printing these tiny, nifty storage containers, threaded to match bottlecaps on common products.
They come in three variants: Normal, knurled and slitted.
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The .stl files were created by Japan-based Thingiverse user Tokyo Bird and are available here, though I'm not sure if Japanese-market bottlecaps are threaded to the same dimensions as the ones in the 'States.
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