Mercury - Weather Cap SmartCraft Junction Box Protection 8M0210754. Overview. Open ports on a junction box can let in moisture, dust, and the kind of grime that slowly causes connection problems over time. The Mercury 8M0210754 Weather Cap takes care of that by sealing off any unused SmartCraft junction box connection points. It keeps the box clean, dry, and protected, helping the network stay stable even after long seasons on the water.. Key Features. Protective Cap - Seals unused SmartCraft ju