Have it engraved under a ring on your finger.
Get a trusted jewleller to do it and never give him or let him copy the key / phrase and make sure you are present when they do it.
Simple, always on your person and if on a wedding ring, never comes off.
I got a better one:
Write half of the private key on your ring, and half of if on another ring that a family member keeps.
A thief would need both of the rings to steal your coins.
And you should of course have "backups" of the rings in case of loss: keep the backups at two different bank vaults.
You should also go to two different jewelers to engrave half of the private key, none of them have the full key so they can't take your coins, unless they know each other and collude to steal your coins.