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The lower and the upper part can get screwed together ( close left turn open right turn). An O-ring in the middle - printed in flexible PLA - ensures a waterproof sealing.
The whole in the top allows to pull through a string to connect it to a key-ring or a carabiner - or you can hang it from a necklace.
For € coins you can print it in 1:1 size. Using it with Swiss Francs or other currencies with larger coins, you would need to size it 1: 1.3 or 1.4.
Sizing it to the double would allow to use the egg also for larger objects like a car key you might want to protect agains water.