Metal box will eat up weight capacity of your platform.
For heavy weight, get a receiver Hitch welded to the frame bars ahead of the "bumper", sewage hose storage tube. For light to medium weight, get the reinforcing hooks that bolt to the frame bars to add extra support to the bumper. The bumper tube is thin steel and butt welded to the frame bars, not always too strongly.
I have a factory rear rack. While it rests on the bumper, its pivot hinges clamp to the frame in front of the bumper. It is rated for 250#, IIRC, and part of that weight is the mounted spare tire.