The guys aver at Laylax have released a new trigger guard called the "Knights Type Trigger Guard". The new guard is designed for the M16 series and is dead easy to fit. The front pin area is held in with a spring pin unit that allows it to quickly snap in and out, whilst the back (towards the pistol grip) is held in with a conventional through pin that allows it to pivot.
Made from aluminium the guard is shaped to allow gloved hands better access to the trigger itself and if needed can be flicked down as shown. Price for the new part is 2,800 Yen.