What Rockwool did you use for the block off plate? I see “rockwool” everywhere on here, but haven’t seen which specific one/r-valueI used a different 12" extension I bought from Rockford, if you dig through my install thread (link in my signature) you'll find it. There's a good chance you'll also need an elbow adapter between the bottom of your liner and the insert, but it depends on how your masonry fireplace was constructed.