What constitutes a "sufficient" ground?
@rdtsc provided a link to the manual which covers what to do with the grounding. You are going to connect DC negative to chassis ground with heavy gauge wire (see manual), and you are going to have to decide what "chassis" means in your case. Maybe if this box of yours is metal, you can just bond the chassis ground to the metal box. But this is not really the important part. More importantly, make sure you use a GFI somewhere between the inverter and your load. Inside the inverter, since it is a UL 458 inverter, there is a relay which connects green wire to neutral only when the inverter is supplying power. When it is in standby or pass-through mode (when you supply AC to the inverter so it can recharge its batteries) the connection from neutral to GND is open. In either case, there is exactly one place where GND is bonded to neutral. Your 12 battery is not a shock hazard. It stores an impressive amount of energy, and can be a fire hazard if there is a short, so make sure you have proper fuses. But if you screw up the AC grounding, you can introduce a shock hazard. Luckily it is not that complicated. Just read the manual again.