This CellarScience Bung has a ridge to keep it from getting sucked in and self-sealing vent holes to let air out but not in. They have a gentle taper, so they don’t slip out during use.  They are also very easy to clean and sanitize. Replaces Stopper Sizes #8, #9

• Stopper Measurements: 1.36" x 1.89" (34.5 mm x 48 mm)
• The flapper stopper protrudes roughly 2.5" from the stopper to allow for easy assembly

Other available sizes include the Breathable Carboy Bung and the Breathable Barrel Bung. All three size options can be purchased in one complete bundle with the Breather Bung Pack.