Most secure way: Wipe the private key (and all of its copies) and then commit suicide. However, I'd not recommend that.

You could instead consider so stash a cold storage in some creative way. Like a weather baloon in orbit around the earth, with an expected drop-off in some future time/location.

Another way is to encrypt it with a weak cipher expected to be obsolete within some future time due to Moores Law, but that's taking risks...