The ornamental design for an endcap for a support rod, as shown and described.
FIG. 1 is a front perspective view of an end cap for a support rod in accordance with our new design;
FIG. 2 is a front elevational view thereof, a rear elevational view being a mirror image thereof;
FIG. 3 is a top plan view thereof, a bottom plan view being a mirror image thereof; and,
FIG. 4 is a left side elevational view thereof.
The broken lines in the figures are for illustrative purposes only and form no part of the claimed design.
The right side elevational view of the end cap for a support rod is not shown in the drawings or described in the specification and, therefore, forms no part of the claimed design.