Do not mix 120 volt AC with 12 Volt DC in the same box. And yes the 120 should be terminated in an approved electrical box. Use romex wiring with approved connectors or flexible conduit from the box into the Circuit breaker panel. I would use 14 and install a 15 amp circuit breaker. Also use and approved cover to mount the meter.
Please not the 120 volt meter is NOT UL listed or approved.
If you some of you are in doubt about the mixing of low voltage and line voltage why do you thank the Circuit breaker is separated from the 12 fused section.