I wouldnt worry about the wood, even if it was touching the muffler. I drive a natural gas trash truck, and everyday something falls on the turbo and lights on fire, poor design on the engineers part. The paper and cardboard lights immediatly, and usually vanishes in less than 30 seconds. I did have a 2x4 land on and get stuck propped on the turbo once. It never lit on fire, turned black, but never smelled smoke from it like I do the papers and cardboard. And the turbo is usually hot enough to glow. I think near the muffler you'll have no problems, especially if its not touching.
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