FIG. 1 is a front perspective view from the top and left side of a portion of a mask frame according to an embodiment of my design;

FIG. 2 is a front perspective view from the bottom and left side thereof;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a left side view thereof;

FIG. 6 is a right side view thereof; and,

FIG. 7 is a top view thereof.

Where included, broken lines showing environmental structure are for illustrative purposes only and form no part of the claimed design. Dotted lines indicate the border between claimed and unclaimed subject matter and form no part of the claimed design.