The ornamental design for a cap for fluidcontainer, as shown and described.
FIG. 1 is a front isometric view of a cap for fluid container showing the new design.
FIG. 2 is a back isometric view thereof, with the cap for fluid container depicted upside down.
FIG. 3 is a top plan view thereof.
FIG. 4 is a front elevational view thereof.
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is a back elevational view thereof, with the cap for fluid container depicted upside down.
The broken lines and areas adjacent to shaded parts of the article depict portions of the cap for fluid container that form no part of the claimed design.