The ornamental design for a movableplatform for a mandolin slicer, as shown and described.
FIG. 1 is a front, right side perspective view of a movable platform for a mandolin slicer, showing our new design;
FIG. 2 is a right side elevational view thereof, the left side elevational view being a mirror image thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines shown in FIG. 6 of the movable platform for a mandolin slicer is for the purpose of illustrating environment only, and forms no part of the claimed design.