chimney and new insert plan. acceptable?

Good Day to everyone. I've enjoyed reading through several pages of posts. I started with a couple of questions but after reading I now have about a million but I'll stick to the original couple. I have removed an old (30 years) Country Flame insert from what I supposed was a fireplace. Evidently the "fireplace" was built with an insert in mind so there are no firebricks, damper, etc. Just kind of a big hole in wall built with common brick and with a 9 inch flue opening in the top. There was never a chimney liner and the old insert was just venting into the firebox. Because the opening is so large I want to reduce the size by adding brick around the openings sides and top. This is for aesthetic reasons because my wife doesn't want a huge insert surround. All the brick surround will be tiled before the new insert's installed. Also, since the hole is so large I plan to fill the sides, back and top with plenty of insulation. Especially since it is an external chimney. Is this plan acceptable? Since I only have a 9 inch flue opening should I include a blocking plate as well on the top? Maybe made with durorock since I'll probably be using that with the insulation anyway? Haven't purchased a replacement yet but am leaning toward a Jotul C350. Thanks, this is a wonderful resource. Mike
the hole in the wallflue opening on the topold Country Flame insert

thanks for the feedback bholler. grisu, I thought about that and if the "hole in the wall" looked half as good as the one in your picture I would consider it strongly.

I read the thread from mellow and that;s exactly what I planned to do. Instead of painting the durock would there be any advantage to facing it with shiny aluminum? I know shiny aluminum is used in various places like attic roofs to reflect heat. Might not apply for this application but just curious.

Since I would be starting from scratch with the insulation/durock I thought maybe of wrapping the durock with heavy foil. The foil would wrap around the edges of the durock and be trapped by the pressure of the insulation, Not real durable sounding I know but once the insert is in place it shouldn't have to be. This is all new to me and I was just kind of thinking out loud.