Mine has the plastic horn on the airbox removed, plus a second hole has been made in the side of the airbox facing the engine. Fitting a second battery causes some impedence to airflow into the box, so a second hole improves matters, alternatively fit a snorkel and the potential problems of water/mud entering the airbox is solved.
You can either wire them in parallel via a relay on the vehicle charging circuit, so that the second battery only connects in circuit with the starter battery when the engine is running. Make sure you fit a large enough fuse in case one of the batteries fails, which would cause loss of both and probably and engine bay fire.
Alternatively you could purchase a digital charging device as an add-on that looks after the charging of the second battery independently of the vehicle charging circuit, OR you could replace the vehicle charging circuit with another digital device to manage independently the two types of battery you'll be fitting, starter and leisure. On the better units if the digital circuitry fails it defaults back to the vehicle charging circuit.
None of the digital circuitry should attempt to charge the second battery if the starter battery is at low volts.
