I made my own stainless firewall cover out of brushed stainless steel. I too didn't want a mirror polished surface in the engine compartment. You can see the fabrication about 1/2 way down this page.
http://www.banzairunnerpantera.com/adventures.htm
I left the stock asbestos insulation on the engine side of the firewall and lined the back side of my new stainless cover with Dynomat, then screwed it to the firewall, sandwiching the original asbestos.
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