Cobraone

Any wood stove or fireplace store sells fiberboard a heat resistant board that could be cut and attached to the wood your concerned about. Your L&L manual describes wood over a period of time of low heat that looses moisture can flash at lower temperatures, I would buy a ir temp gun ( harbor freight sells them cheap} and find out the exact temp the ductwork is running at, it’s hard to be over concerned when dealing with fire safety, I hope this helps! best regards!