hello, a couple of weeks ago I posted a question regarding raising the main rectangular duct trunk up 4 inches to gain headroom in my basement. Thanks to all who responded. Raising the main trunk will be the easy part. It's the transition piece that I am unsure of... . I will need to get some sort of galvanized 4" riser. If I make one myself, how long should the riser be? In other words, to help the airflow, I don't think I should use an elbow. My thought was the riser should gradually taper up 4" over a couple of feet. Also, if I screw this up, any ideas what it would cost a sheet metal guy to make one of these? The duct is rectangular 18" x 10". . Thank you for your time.