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.
Easy To Make Stone Art - write message on rock with pencil, Use Dremel w/grinding stone, run over pencil mark once to create groove, go over three times or until you reach desire thickness. - now I just need a dremel.