I claim the ornamental design for a cubical folding box, as shown and described.
Figure 1.1 shows a perspective view of a cubical folding box;
Figure 1.2 is a perspective view of a partially-unfolded embodiment thereof;
Figure 1.3 is a second perspective view of a partially-unfolded embodiment thereof;
Figure 1.4 is a third perspective view of a partially-unfolded embodiment thereof; and
Figure 1.5 is a plan view of an unfolded embodiment thereof.
The dashed lines in Figure 1.5 are included to represent valley fold lines for the shown cubical folding box; the dashed-dotted lines in Figure 1.5 are included to represent mountain fold lines for the shown cubical folding box.