I did a much easier fix. I took 2 metal straps about 1 1/2 inch wide, drilled a hole in each end. Took 2 carriage bolts & welded 1 in each end of one strap. Located the spot under the rack where the second strap would contact the metal outer edges of the rack & drill two holes from the underside of the top case that lined up with the holes in the strap. I then put the strap with the bolts in from the inside of the top case (bolts exiting the top case to the bottom) & installed lower strap. The only draw back is the strap had to be removed to remove the top case. This winter I may built a bracket like you did. It looks like a much better fix.