The ornamental design for an outercovering for a percussive high frequency chest wall oscillation system, as shown and described.
FIG. 1 is a front perspective view of an outer covering for a percussive high frequency chest wall oscillation system in accordance with the present design, as viewed from a top right corner thereof;
FIG. 2 is front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines showing the remainder of the outer covering for a percussive high frequency chest wall oscillation system are directed to environment. None of the broken lines form a part of the claimed design.