A seat box assembly and method of fabricating utilizing a drop-in webbing assembly secured into a main box and back frame assembly. The webbing assembly has a rectangular frame with elongate members and shorter members and a central girder with a cutout extending between mid-portions of the elongate members. The central girder attachable to the seat box and a central vertical frame member of the back frame assembly. The elongate members having stub portions that fit into slots in the main seat box. A method of repair includes replacing a damaged drop-in assembly with a different drop-in assembly.