No way for closing the mouth:
This metal O-Ring gag comes with a head harness that is made of 3mm thick and 20mm wide rubber straps, and fits perfect.
The inner diameter of the gag is 26mm, the outer diameter is 46 mm.
For more safety the buckles of the harness can be equipped for locking with padlocks.

The harness fits for a forehead circumference of 52cm to 62cm.