Here's a different take on the fence post cache; instead of a micro hidden under a metal fence's cap, it's a tube hidden in a sawn off fence post. Just remember, you can't do this to a post you don't own or don't have permission to modify.
This is such a fun idea! or use dremel tool on a real brick. Perhaps a small real Lock & Lock could be used instead of a film canister. Would be watertight and more fun to be able to leave signature items and travelbugs.