Take a strip of metal and carve your (unencrypted) private key into it and bury it somewhere.
You can also take a piece of polished granite to carve your private key, this way it will not be detectable by a metal detector.
Recommend using a non-ferromagnetic material with good corrosion resistance, like aluminum or high-quality stainless steel. If you believe hand carving to be intimidating or too time consuming, you can buy a
metal punch kit to make the impressions (with a hammer of sorts).
Once complete, add clear coat for some initial resistance to the elements. Real paranoia would be doing this with 2/3 key multisig in 3 different plots of land, or with backup copies, etc.