Are there any members here that are aware of a supplier of 2 pc corner extrusion as seen in this youtube video? These are from COMPOSITE PANEL TECHNOLOGIES but as with some other suppliers, theses parts are not available 'a al carte' for home builders.
At one time they sold them but they were really expensive. I'm not sure what I think of that wedge shaped void.
For DIY I would probably do the rabbet joint and bond some thin L aluminum extrusion over the outside and on inside corners