Almost finished this project now. I was going to call it finished as far as the structure but I gave in and decided that I want to install fire-proof sheets inside it. That will be a job for next week.
I’ve put thread locking nuts on many of the crucial bolts just as extra support
The front door of the container coming together. This is definitely a V1. An improved version for example could simply be hidden better, rather than thinking about the structural rigidity of the container and needing to bolster any weak links. Choosing an inconspicuous place to store this has the advantage of not making it look like a target for thieves with its solid construction. None the less it’s not easy to move and should do the job to a good spec.