I just came in again and thought I'd look for emails before retiring. A few days ago I added a three-foot extension of 8 inch rigid above the chimney, and it's very sturdy and looks good. It really helps to lift the smoke, invisible or visible, well above the roofline.Does your manual actually require 6” to a concrete wall in the rear or is it a “clearance to combustible”? There’s a difference.
If not prohibited by the manual due to rear clearances, I would prefer not to have the additional 90 and horizontal section on your relatively short chimney. As I recall you only have a 16’ stack and two 45s subtract from that to make your stack less than the required 15’ minimum.
Again, thanks everyone for your advice and wisdom of experience.