The ornamental design for a square fencepostlightfixture, as shown and described.
FIG. 1 is a front view of a square fencepost light fixture showing my design. The back view is a mirror image of the front view and therefore, has been omitted. The broken line showing of the globes is for illustrative purposes only and forms no part of the claimed design.
FIG. 2 is a top view thereof; the broken line showing of the electrical socket is for illustrative purposes only and forms no part of the claimed design.
FIG. 3 is a bottom view thereof; the broken line of the socket and electrical wires is for illustrative purposes only and forms no part of the claimed design; and,
FIG. 4 is a perspective view of the square fencepost light of FIG. 1. The broken lines showing of environmental structure in FIG. 4 is for illustrative purposes only and forms no part of the claimed design.