Coins hidden in crown caps
Some of the existing coins fit very accurately into crown caps. You dont need to drink a whole beer crate every day, just wait and plan long enough in advance. Collecting crown caps for the right occasion is a hobby and was very popular with kids some time ago. Perhaps you can ask them and find some special kind of crown cap that is not available anymore?
If it comes really bad you might have to take some crown caps that are already a little bit desolated, but you can get them back into shape and fixate the coins with glue.
Even nicer it would be if you can organize a vessel in which you present your crown caps!