Common, inexpensive face masks are often used to deliver oxygen to patients. They are usually held in place by elastic which covers the nose and mouth with oxygen delivered through a plastic tube attached to the base of the mask. There is no attempt to seal the mask and face. Masks that seal preventing entry of external air are more complex, difficult to fit, and prone to leakage (eg a seal is difficult with a beard). CPAP devices used for sleep apnea, and the mask used in ther DeBreather end of life device (see Dec 2019 update) are of this type
The Exit Bag produces rapid loss of consciousness by ensuring that NO oxygen is inhaled. To achieve this using a mask a perfect seal would need to be maintained till death. Even with a well fitting sealing mask this is difficult, as the muscles and contours of the face change as consciousness is lost. Attempting a hypoxic death using a sealing mask is risky and NOT recommended.