Try searching sissy bars on ebay. Murrayeliminator and I bought used 1970's/1980's motorcycle sissybars off of there and retro fitted them to our JcPenneys Dusters. The Slingshot does have a unique bend, but maybe you could find something on ebay that you can modify to work? Stay away from the schwinn bicycle banana seat type sissy bars. The metal is too weak when trying to bend it.
My avatar and "view my photos" shows the 1980's honda sissy bar I used on my Duster.