When you sit in a seat and it fits your body type just right, you feel snug in there. Likewise, when you wear your mask and make sure to fit it so that it stays close to the contour of your nose, your lower jaw, and your chin just right, it feels snug. There are no big gaps around your facial features. It is as if it was designed just for your face. Otherwise, it doesn't perform its function that it was meant to.

If you use a feminine term for mask, make sure to adjust the modifier:
مُنضبظة الوضعية، مُحكمَة التثبيت

Also,
موضوعة بإحكام، مُثبَّتة بانضباط