I think it was 4mm plate I bought from Ebay dirt cheap, a few hours in the garage with a hacksaw, file and drill. Draw up a template on paper, copy over to ally then rough it out with the hacksaw, dress it with a file. Make the folded edge with a vice and hammer or shifter to bend the plate. Mark out the cut out in the middle, drill holes round it enough to get a saw blade in and cut it out, dress it with a file, a quick coat of paint and your done.
Dead easy and uber cheap!!