How to block gap around chimney at attic floor?

I've got a 200 year old center chimney cape, that has a warm attic space.
Mostly, it appears that warm air is rising into the attic through a 2 inch
gap between the floor and the brick chimney. Previous owners had a few
wooden shingles and some fiberglass insulation there as an attempt to block
it. We've got the venting pretty well covered, but a surprising amount of
warm air is coming through that gap!

What is the preferred way to block this gap? I'm thinking of getting some
sheet metal to cover the gap ( as found with a web search ). The chimney
is lined with a clay liner and one flue is used to vent the furnace but I'm
thinking the biggest concern would be when we light the fireplace a few
times a year.

I'm guessing that this problem is very common, and an effective solution is
well known? Any thing better than sheet metal?

Thanks for any info on this.

-Johny B.