This is kind if a modern version of the Fabergé eggs. It is small enough to hang it with from a necklace a or attach it to a keyring. The egg is desi...gned to store coins or - in an enlarged version - other objects like a car key.
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.
3d model print parameters
The shells are printed in ABS, the O-Ring in flexible PLA.
I printed it with 1.5 mm shell thickness and 30% fill density.
To print it I would ad...vise using a brim and "touching build plate" support.