We claim the ornamental design for a manual carton cutter, as shown and described.
FIG. 1 is a perspective view showing the new design.
FIG. 2 is a left side view showing the new design.
FIG. 3 is a bottom view showing the new design.
FIG. 4 is a top view showing the new design.
FIG. 5 is a right side view showing the new design.
FIG. 6 is a front view showing the new design; and,
FIG. 7 is a rear view showing the new design.
The shading lines shown in the drawings represent the approximate three-dimensional contour of the design and are not intended to indicate surface decoration.