The design for a cardboardtubeplug, as shown and described.
FIG. 1 is an isometric view of the cardboard tube plug embodying the present ornamental design;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a rear view thereof, showing the claimed cardboard tube plug in an upside down position;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in FIG. 7 represent the three dimensional configuration of the four innermost circular surfaces, and form no part of the claimed design.