The ornamental design for the door for a shower cubicle, as shown and described.
FIG. 1 is a front perspective view of the door for a shower cubicle invention;
FIG. 2 is a front perspective view thereof with said door in the open position;
FIG. 3 is a rear perspective view thereof showing the side walls broken away for clarity of illustration;
FIG. 4 is a front elevation view thereof;
FIG. 5 is rear elevation view thereof;
FIG. 6 is a left side elevation view thereof;
FIG. 7 is a right side elevation view thereof; and,
FIG. 8 is a top plan view thereof.
Structure that is not part of the claimed design, but is considered necessary to show the environment in which the design is associated, is represented in the drawings by broken lines.