The HMF Spark Arrester comes with every HMF exhaust, installs in seconds, and is U.S.F.S. Forestry approved. It's designed to keep heat and debris from exiting the exhaust into the woods.
Check your spark arrestor screen from time to time to see that it is free flowing and not clogged.
The easiest way to clean the spark arrestor is to remove it from the exhaust then soak the spark arrestor in oven cleaner. Wait for a few minutes, then clean the remaining residue with a wire brush. Keeping the spark arrestor clean will increase exhaust flow and performance.