This is a story about an 88 year old friend of mine. He was a young man during the time of Cuban missle crisis. His father devised a plan to reunite with each other in the event a nuclear exchange occured while they were apart. They hid Mason jars with pencils and paper at their favorite hunting spots around Arizona. The idea was to leave a note with the date and where they were headed. Eventually they would find each other. The nukes never showed and fifty years went by.
His father having passed away decades earlier, Charly hobbled out to an easy location and climbed up to the hiding place for one of the jars. Inside the jar was a fifty year old letter from his father. I doubt his dad thought it would be fifty years before the note was discovered. It was very touching to hear the story. Perhaps this idea could work for you.