The ornamental design for a coverpanel, as shown and described.
FIG. 1 is a perspective view of a cover panel showing my design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view thereof shown in the operating condition and the housing shown in broken lines represents environment only and forms no part of the claimed design.