I made my own by cutting out the cardboard waffles in the OEM and replacing with Unifoam that I cut to fit. And fit tightly. Then I oiled it good. You can purchase a large sheet of Unifoam at most shops. Motorcross racers use it. It helped the engine breathe and upped my mileage a couple of miles. Then I replaced OEM muffler with a GPR from Italy. The combination helped mileage and performance. Only now with the engine breathing more, the rpm has gone up at highway speeds. When It hits red line, I back off slightly and CVT drops a gear, then I give it more gas and off we go. Now I am considering a Malossi variator and clutch. Other threads talk about how well Malossi performs.