Coffin Junction Box 2.5mm sq. Size: 75 x 40 mm Powermaster 2.5Mm Sq Coffin Junction Box 75Mm X 40Mm | 1798-18 The Powermaster Outdoor Junction Box - Coffin Shaped is a waterproof container for electrical connections, usually intended to conceal them from sight and deter tampering. Made of durable white plastic, this round junction box is suitable for domestic and commercial use and is ROHS approved.