Abstract: An open-top cargo box for a truck trailer that is selectively coverable with bow-supported tarpaulin system. The cargo box has an interior cavity defined by front and rear walls connected together by a pair of longitudinally extending side walls. Each side wall has an upper horizontal surface defining a plurality of spaced-apart apertures therein. Each aperture has a first cross-sectional shape and a first size. The bows that support the tarpaulin include vertically oriented ends, each of which has a second cross-sectional shape and a second size. The second shape and size of the ends of the bows is smaller than the first shape and size of the apertures. Consequently, if the side walls of the cargo box spread apart under pressure exerted by a load carried in the cavity, the ends of the bows slidably move within the apertures in the side walls in a direction substantially perpendicular to the longitudinal axes of the side walls. The side walls may be reinforced proximate the apertures.