This is a 7mm:1ft scale model of a British Railways (actually pre-nationalisation too) DX coded open container. In real life the DX container had removable sides rather than a door in the side. It fits inside my GWR design O35 medium open goods wagon and comes with a sprue of two stretchers to fit across the top. The stretchers have angled holes near the ends and a central vertical hole. Clean out the holes with a drill and fit a curved bar made of wire or thin plastic bowed between the ends, and a support strut under the middle. Work from photographs to get the proportions.