Ceramic Fiber Components
Ceramic fiber was originally developed for the space program. It is expensive, but will not melt in the conditions found in a wood-burning device. We sell this material by the square foot. Use it to insulate some parts of your device and channel the heat to another area, as desired.
For example, we use the ceramic fiber blanket to insulate the chimney flue liner from the heat of the exhaust coming out of the heat riser in the castle build. Chimney flue liners have to be insulated from temperature changes more than 50°F per hour.
You could use that same blanket to insulate some or all of your steel barrel and cause the heat to radiate into your design later in the exhaust flow.
The gasket paper will form a gas-proof barrier between dissimilar materials.