The ornamental design for a fencepostbase, as shown and described.
FIG. 1 is a top view of a fence post base, the bottom view being a minor image thereof;
FIG. 2 is a left side view thereof, the right side being a mirror image thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a perspective view of the design of FIG. 1;
FIG. 6 is an exploded perspective view of the design of FIG. 5;
FIG. 7 is a perspective view of a second embodiment of my fence post base wherein the second variant has screw holes located on each side of the upper portion; and,
FIG. 8 is an exploded perspective view of the design of FIG. 7;
The broken lines in FIGS. 6 and 8 are for illustrative purposes only and form no part of the claimed design.