u/Chance-Following-665

I need to extend this ductwork about 3' to the next set of joists. I was expecting 6" or 8" but this one seems to be 7". It's used mostly for air circulation and sometimes forced air heat, so it doesn't need to be insulated. It's between the first floor and the second floor. What's the best way to extend the ductwork, flex, metal...?