The ornamental design for a sinkdrainboard, as shown and described.
FIG. 1 is a perspective view of a sink drainboard showing our design;
FIG. 2 is a side elevation view of a first side;
FIG. 3 is a side elevation view of a second side;
FIG. 4 is a top plan view; and,
FIG. 5 is a bottom plan view.
The broken-line disclosures in the view represents portions of the sink drainboard in which the claimed design is embodied, but which forms no part of the claimed design.