One of the quickest ways to get “custom” parts like this is to see if there are other builders in your area that might have their exhaust system sitting on the shelf, waiting for installation - but that installation is months (or years) down the road. Borrow the part from them and then re-supply them from the manufacturer. You get the part right away, and they aren’t out anything. This is a great value to having a community of builders when it comes to special kit parts.
Paul