The ornamental design for a thermalinsulator for a beverage container, as shown and described.
FIG. 1 is a front elevation view of a thermal insulator for a beverage container showing our new design, each of the opposite side, the right side, and the left side being a mirror image of the shown;
FIG. 2 is a top plan view thereof.
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a top isometric view thereof;
FIG. 5 is a bottom isometric view thereof;
FIG. 6 is an in-use right side elevation view thereof; and,
FIG. 7 is an in-use bottom isometric view thereof.
The broken lines in FIGS. 6 and 7 of the drawings depict a beverage container and a koozie, respectively, as environmental subject matter only and form no part of the claimed design.