An attachment clamp for a utility vehicle cargo box with a cylindrical bar along a top surface of the sidewall. The attachment clamp includes an inner plate positioned inside the sidewall, and an outer plate hinged to the inner plate and positioned outside the sidewall. Each of the inner plate and outer plate including a bar gripping element to grip the bar along a top surface of the sidewall. A handle is connected to the plates and pivotable to move the inner and outer plates together to a clamped position around the bar or apart to an unclamped position. The handle may include a lobe that cams the plates together and enters a recess in the outer plate in the clamped position, and a spring urging the inner and outer plates apart toward the unclamped position.