I just finished doing this to my Papa bear. Measured out to 17" x 8" using 1/4" steel plate. I just hope it doesn't affect the draft. Being that i have three elbows by the time it reaches the chimney, my draft isn't the greatest. I'm most worried about getting blow back due to cooler flue temps. Can anyone tell me what the best temp to keep the flue pipe at? I can always take it back out if things don't go as planned.