Plate or rock wool?

I had my insert and chimney cleaned last month for the first time and the sweep said I needed to insulate my chimney. I also had regular insulation up inside above the insert. Found some that had melted just a little. So I bought 40 # of rock wool from the company someone so kindly posted about.

My liner runs up where the damper used to be inside my heatilator fireplace and has some space above and on the sides.

Now my main question is, should I try and get a block off plate made and installed, or just stuff rock wool everywhere I can around my insert?

Thanks
Shipper