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.
Fake electrical box with a hidden key - the key it attached to the small grey cylinder and this was hidden down inside the post. The grime and mould spots on the electrical box were actually artfully painted on and it also had GC stencil on outside.