I claim the ornamental design for a bottle cap fixture, as shown and described.
FIG. 1 is an end view of the bottle cap fixture;
FIG. 2 is a top view thereof;
FIG. 3 is a side view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a right front perspective view thereof; and,
FIG. 6 is a right front perspective view of the bottle cap fixture in a mounting position on a bottle.
In the drawings, the evenly-spaced broken lines show portions of a bottle and a bottle cap fixture that form no part of the claim; the dash-dot-dash lines represent boundaries of the claimed design and form no part thereof.