Honestly the only 100% secure way to store something is to write it down on physical paper and then guard that.

The problem with that is usability. If I need to sign a transaction that security is lost the moment the key is entered into the signing computer.

Proximity seems to be key to most of these attacks, so maybe physically excluding any possible eavesdroppers, and adding noise sources would create a shell equivalent to guarding that piece of paper.

I also anticipate gathering old/very limited electronics that can be visually inspected or don't have extra capacity to run malicious code to allow auditing the mechanisms of computation.