Cover their face in silicone... Oxballs Guard Gag fits seamlessly over the mouth and face, made of soft silicone. The colored mouthpiece can also be removed for entry. The back is fitted with an adjustable leather strap, whose buckle closure lets you get the right tightness.
The Oxballs Guard Gag stifles the voice, but allows easy breathing. Its silicone face cover will stay cleaner and smell better over time than leather or neoprene, due to the nonporous material. Just wash your Guard Gag after each use to keep it good as new.