Dr. Teddy Woll of Daimler-Benz, in 2014, wrote that the entire exhaust system of an internal combustion vehicle could be thermally insulated to facilitate enclosing the entire length of the underbody.
One would need to use commercial/industrial insulation products as Knauf provides.
They're not a 'retail' product that a 'civilian' can walk into a store and purchase.
With a 'business card' they might let you through the door.
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Short of that, you might be able to just rely on metal heat-shields/ multiple heat shields as you may see above and below catalytic converters.
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American ICBM missile silos use sixteen concentric metal heat shields around the missiles to stabilize temperatures. The company in Dallas that I used to work for was a contractor to the Pentagon for working around technology like that.
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You need about a half-inch air-gap between shields. And some can be attached to the upper face of the coroplast if you can't do hangers from the exhaust system above.
I was able to use 1/8-inch thick Luan door skin on the Spindletop CIVIC CRX by doing this.
I don't see why the coroplast would be a problem.
