A whistle mute is easily made by shaving a matchstick down and using it to divide the windway in such a way that you narrow it at the fipple opening. This doesn't affect the intonation, often improves the tone, and still lets you play the instrument with some expression at quieter volumes. I use it mostly on a very loud low-D, which means I can play the thing despite diminished puff on account of asthma.