The ornamental design for an stackableadultboostercushion, showing my new design.
FIG. 1 is a three-quarter front view of the stackable adult booster cushion of the present invention;
FIG. 2 is a top view of the stackable adult booster cushion of FIG. 1;
FIG. 3 is a side view of the stackable adult booster cushion of FIG. 1;
FIG. 4 is a rear view of the stackable adult booster cushion of FIG. 1;
FIG. 5 is a front view of the stackable adult booster cushion of FIG. 1;
FIG. 6 is a perspective view, showing the stackable adult booster cushion in a stacked position; and,
FIG. 7 is a side view of the stackable adult booster cushion in place on a chair adjacent a table with a short individual on the cushion.
The broken lines in FIG. 6 showing the additional cushions and other structure are for environmental purposes only and form no part of the claimed design.