When I built by 1/96th scale shuttle stack, I used embossed paper towels stiffened with a 50-50 mix of water and white glue, then shaped, for the flexible insulation curtain that surrounds the RSRM nozzle. There is also an insulation ring that surrounds the outer edge of the flexible curtain, and I also used strips cut from stiffened paper toweling for that, as well as the blown-on insulation rings on the bottom segment of the RSRM.
The photos aren't the greatest but they give you an idea of what I'm talking about:
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