The ornamental design for an adjustablecoolerinsert, as shown and described.
FIG. 1 is a front perspective view of the adjustable cooler insert in a slidably adjusted position;
FIG. 2 is a top view of the insert;
FIG. 3 is a bottom view of the insert;
FIG. 4 is a left side view of the insert;
FIG. 5 is a right side view of the insert;
FIG. 6 is a front side view of the insert; and,
FIG. 7 is a back side view of the insert.
The broken lines in the drawing depict drainage openings, leg attachment features and indicia bearing medallions that form no part of the claimed design.