To do a proper masonry passthrough it isn't a 12" square. It is 12" past the thimble. So the square would be 12"+6"+12" or a 30" square. You are doing a non-combustible wall so that isn't required but I would fill around it with mineral wool insulationIt is a new (display model that had one tiny burn allegedly) Nectre N550. Clearance to combustibles is 10” as per the label but i’m looking to put it around 7” since it’s 100% non-combustible wall.
That said, i’d rather not see the stainless portion, just the black stove pipe and trim ring. Based on Bhollers 1.5”-2” suggestion i think that’s possible. So my current plan is to get the metal framing in place and measure, then figure out my rough pipe length needed. Any comment on whether to pack the thimble with cement in a roughly 12” square supported by the framing?