Should the signing device be bigger, harder to break, or what? I think imprinted in steel is as good as solution as you can have for physically securing something. At the end of the day, physically storing wealth has always been a challenge. It’s going to take some invention to solve that has eluded mankind forever. This is a meat space problem, not a digital one. The alternative is to trust third parties with it, and you are just trusting them to secure it properly. I know which option I’m choosing.