Roof top Chimney transition plate for 36” extension.

[switepine]

I have been struggling with adding a recommended 36” insulated stainless chimney extension and am struggling with the transition plate idea? I will post a few photos. (I know I have a creosote problem not the issue here)
1- what exactly is this “6DT-AP14x17 Transition Anchor Plate 14" x 17" “ my tile chimney is only 8”x8”
I assume this is to seal off the top of chimney with a airtight support and make the connection to the
“DuraTech 6" x 36" Chimney Pipe / 6DT-36. “
2- why are these so expensive? See quote below also.

 

[DAKSY]

I believe you will have you take a grinder with an abrasive wheel & bring the terra cotta tile flush with the chimney crown. You should be able to fasten the anchor plate to the crown after that.