4" Attic Insulation Shield is used wherever the chimney enters an open attic space.
Acts as an insulated shield to keep blown or blanket insulation from filling in any framed opening or contacting the chimney.
A - 6-1/8" B - 12" C - 10" for your dimensions
This is another product to consider for your installation. https://flameinnovation.com/product/4-firestop-radiation-shield/ This is a great alternative to some other products. For example, you need to keep combustibles away. In short, this is one of your choices.
To Clarify, go to this site to see this and other products in the HT line to see how they are installed. For example, this article will give you and idea of what to pick. https://www.selkirkcorp.com/en/
4" Attic Insulation Shield is a well built product that must be used for through ceiling penetrations. Or, something like it.