The ornamental design for a thread-on beverage container cap, as shown and described.
FIG. 1 is a top plan view of a thread-on beverage container cap showing my
new design. FIG. 2 is a bottom plan view thereof. FIG. 3 is a right side
elevation thereof. FIG. 4 is a left side elevation thereof. FIG. 5 is a
front elevation thereof. FIG. 6 is a rear elevation thereof; and, FIG. 7
is a perspective view thereof.