The ornamental design for a cake support structure, as shown and described.
FIG. 1 is a front elevational view of a cake support structure showing my new design, the rear elevational view, left side view and right side view being the same thereas;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof; and,
FIG. 4 is a perspective view thereof.
The broken lines showing cake layers in FIG. 1 is for illustrative purposes only and forms no part of the claimed design.