The ornamental design for the cake stencil holder, as shown and described.
FIG. 1 is a perspective view of a cake stencil holder showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a side elevational view thereof; and,
FIG. 4 is a bottom plan view thereof.
The broken lines in the drawings of stencils and openings in the cake stencil holder are included for the purpose of illustration and form no part of the claimed design.