I'm not a firework expert, but with this type of project it's always good to have redundant safety systems. You didn't mention any physical interlocks on this unit. I would include at least one on either the main power or the signal out of the control unit. You can get a key switch/key lock fairly inexpensively (don't let the first few items listed in the link scare you ;), Sparkfun also has some though they don't have voltage and current ratings). Of course you'd generally have to pay a bit extra for non-generic keys, but that likely isn't a requirement for this project. Otherwise, the most basic are similar to what you'd find on bike locks or locks on IT equipment.