There are times I want my slave gagged, and there are times I want my slave's mouth agape.
When the latter is the choice, I stick to the leather covered Rim Ring. A metal ring is wrapped with strips of leather and attached on either side to a buckling belt. I merely insert the ring into my slave's mouth after demanding to open wide, and then secure the ring in place with the buckle. The wrapped ring prevents unnecessary trips to the dentist by keeping the teeth clear of the metal surface. When the ball gag won't do, the Rim Ring is the logical choice.