The ornamental design for a top panelretainer, as shown and described.
FIG. 1 is a perspective view of a top panel retainer according to the invention;
FIG. 2 is a side elevational view of a top panel retainer according to the invention; and
FIG. 3 is top perspective view of the end portion of a top panel retainer according to the invention, with the other end portion being identical thereto; and,
FIG. 4 is a bottom plan view of the end portion of a top panel retainer according to the invention, with the other end portion being identical thereto.
The article shown broken away to show indeterminate length.