The ornamental design for an insulatingsleeve for a beverage container, as shown and described.
FIG. 1 is a front elevation view of a first illustrated embodiment of an insulating sleeve for a beverage container showing our new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a right side elevation view thereof.
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
In the drawings the long dash broken lines in the drawings depict environmental subiect matter that forms no part of the claimed design; the short dash lines depict potions of the article that form no part of the claimed design.