Black Bart II Flue help

[Crocs0]

I have a Black Bart II fireplace insert ( Circa 1982 ) that I have inserted in a heatilator i80 fireplace. I am in need of help to adapt the ~4-1/2" x ~14-1/2" rectangle opening on the Black Bart II to the 11" round chimney flue pipe on the fireplace. The difference in center to center is ~ 6-1/4" with an ~11-3/4" space from the top of the insert to the beginning the of the chimney fluepipe. Any help will be greatly appreciated whether it be an aftermarket purchase or DIY remedy.

 

[bholler]

You need a full liner from the top of the chimney to the insert the fireplace chimney is not adequate.