It's not the air intake that's the problem. These engines are air cooled, the heat sink fins on the cylinders are the only way it has to cool itself. If it was a liquid cooled engine, it wouldn't be a problem because you could just hang the radiator outside the box. Unless there's cool air circulating directly around the block itself, it will burn up. If you want to build something super quiet, you'll either have to spend too much money, or connect the generator head to a liquid cooled engine.