I really do like the Arkel system (at least, the one back from four or so years ago). Simple aluminium flat bar bent into hooks and drilled for the two hitches onto the rack, and another piece of flatbar bent and drilled to form a "butterfly" clip for the centre. Look up Arkel's website that might give you some ideas. You do need a backing stiffener of some sort (either another long piece of alloy flat bar).