Tuff Box is a heavy, one-piece durable polymer shell. Virtually air-tight molded design delivers 100% of air flow. Using an innovative stepped design supporting 3 duct sizes, collars are 1-3/8 in and beaded to hold flexible duct in place while securing strap. TRUaire Retrofit TuffBox Register Box, Small, 6 in Length, 6 in Width, Polymer, Black