The ornamental design for a coverplate for a computing device, as shown and described.
FIG. 1 is a front view of a cover plate for a computing device;
FIG. 2 is a top view of the cover plate for a computing device; and,
FIG. 3 is a bottom view of the cover plate for a computing device.
The broken lines showing of various regions and the unshaded areas therein are for the purpose of illustrating the article and form no part of the claimed design.