It is probably in the nature of things that the lid does not remain 100% accurate to the mussel. We just made each to the edge three holes and close the two shell halves with the help of very simple carabiners to protect the game so before sand cats and rainwater. We also drilled several small holes in the bottom one, so that the water, when you make one that arrives while playing in the shell, also can flow again.
So now we have our own small sandy playground, where I can be making his first experience with sand and water my little good conscience.
Addendum of 07/05/2013
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I hereby subtract a star because of lack of stability. When you move the sand seashell she has broken the side and the circular edge is now sharp and thus poses risk of injury. Very unfortunate, although it was clear from the outset that this article is not made to last forever I had a somewhat longer-lasting resistance expected.