The ornamental design for a cushion for a respiratorymask, as shown and described.
FIG. 1 is a rear perspective view of a cushion for a respiratory mask according to our design;
FIG. 2 is a rear view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a front perspective view thereof.
Where included, broken lines showing environment or structure are for illustrative purposes only and form no part of the claimed design. Where included, dotted lines represent the border between claimed and unclaimed subject matter and form no part of the claimed design.