Make a small hole in a titanium screw, either ingrave the privkey inside that cavity, or insert something durable with the privkey engraved on it within the cavity. Seal it. Now, go and break your arm, insert a few screws to fix it. The screw with the privkey goes along with the other screw. If somebody ever asks, just say you were in a horrible ice skating accident when you tried to qualify for the nationals. Privkey secured. Retrieval might be another matter, but you're in this for the long term, no?