The ornamental design for an insulationdevice for a stemmed beverage container, as shown and described.
FIG. 1 is a front perspective view of an insulation device for a stemmed beverage container;
FIG. 2 is a front view of the insulation device for a stemmed beverage container in an open position;
FIG. 3 is a front perspective view of the insulation device for a stemmed beverage container; and,
FIG. 4 is a back perspective view of the insulation device for a stemmed beverage container.
The broken, dashed lines shown in the various figures are included for the purpose of illustrating context and do not form any part of the claimed design.