The ornamental design for a flat paneldisplaystandbase, as shown and described.
FIG. 1 is a front view of a flat panel display stand base;
FIG. 2 is a back view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a left view thereof;
FIG. 6 is a right view thereof; and,
FIG. 7 is a perspective view thereof.
The broken lines shown in FIGS. 2 and 4 illustrate portions of the flat panel display stand base that form no part of the claimed design. The broken lines in FIG. 7 illustrate environment that forms no part of the claimed design.