The ornamental design for top plate with a coveropening/closing mechanism, as shown and described.
FIG. 1 is a perspective view 1 of a top plate with a cover opening/closing mechanism showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The parts shown in the broken lines is for the purpose of illustrating portions of the article and do not form part of the claimed designs. Dotted and dashed lines represent boundaries between the portions intended for registration as a partial design and the other portions.