A simple clip designed to block the whistle of a recorder. This reduces the volume significantly and the user can still (somewhat) make out different notes. Alternatively, you could shove your finger into the recorder while the player is practicing to achieve the same effect.

Just a heads up: due to how thin the model is, it is very easy to bend when removing it off of the build plate. I would recommending 2 or 3 and giving the spares to your best friend.