Type in motorcycle luggage rack on Ebay and scroll through there are a few suitable ones hidden in amongst them. Alternatively get one water jetted, lazer cut, milled out of aluminium or stainless steel to your design. I used to make them at work out of 3/8 stainless tube bent up with a Rigid tube bender and get the welder at work to TIG them for me. Made allsorts of stuff out of stainless tube.
Regards Martin.