Here is an update of the construction of my lower block off plate. To summarize, this is a 22 gauge piece of sheet steel that I bought at Lowes for $30. I made my plate in two pieces so that it is easier to remove as mine is 46" long. Then I painted it with Rutland 1,200 degree stove paint for $12. It will be screwed into place with 3/16" Tapcon screws drilled into the fireplace masonry joints. Above the plate I am using a 2" thick Roxul Rockboard 60 insulation board which is fireproof and rated to 1,200 degrees with an ultimate melting temperature of 2,100. This stuff is very easy to work with and can be cut with a utility knife. I bought one board from www.atsacoustics.com for $13. This provides an R value of 8.4 for the 2". I have enough left over to put a plug up at the top of my chimney as well. I haven't decided if I need to seal around the perimeter with high-temp silicone. Thoughts?