We claim the ornamental design for a stool base, as shown and described.
FIG. 1 is a perspective view of a stool base on a stool showing a top, left side and rear.
FIG. 2 is a top plan view of the stool base of FIG. 1.
FIG. 3 is a rear elevational view of the stool base.
FIG. 4 is a front elevational view of the stool base; and,
FIG. 5 is a left side elevational view of the stool base, the right side being a mirror image.
The broken line showings of the mounting portion in all figures illustrate portions of the stool base that form no part of the claimed design. The broken line showing of the stool seat in FIG. 1 illustrates environmental structure and forms no part of the claimed design.