A good place to start with is physical security. Install the FortiGate unit in a secure location, such as a locked room or a room with restricted access. This way unauthorized users can’t get physical access to the device.
If unauthorized users have physical access they can disrupt your entire network by disconnecting your FortiGate unit (either by accident or on purpose). They could also connect a console cable and attempt to log into the CLI. Also, when a FortiGate unit reboots, a person with physical access can interrupt the boot process and install different firmware.