Why do the regs require a box?
Childproof?
Why non-ferrous fastenings?
The reason for a thick wooden box is to isolate it from fire. A small fire or ignition source won't immediately light your powder store (for example if your carpets or curtains catch on fire, or if somebody drops a cigarette or match). In a big fire it will eventually light of course, but that will be after a long exposure, at which time there are unlikely to be any surviving people nearby.
They specify wood not steel, so that the box is relatively weak. When the powder inside a box lights, you want the box to break before too much pressure builds up inside. If powder inside a strong steel box lights, it is called a bomb. Better to have the box break apart once pressures inside reach a few hundred psi, rather than reaching thousands or tens of thousands of psi.
I visited a friend in Germany and he showed me where he kept his powder - in a big strong steel safe. He understood that this was wrong and dangerous, and was effectively a bomb-in-waiting in his house, but it is what he is required to do by the inspection authorities. The only consolation is that it is in a location far enough from his family and his neighbours that it probably would be survivable.
Non-ferrous fastenings are likely an overkill in the specification for a small home powder storage box, but the reason is to avoid any possible source of sparks. Ferrous materials make sparks when struck, nonferrous materials don't. Personally I suspect that electrostatic discharge is probably just as much worth worrying about, but for whatever reasons they didn't work that in.