The ornamental design for the spool organizer, as shown and described.
FIG. 1 is a right side elevation view of a spool organizer showing my new design in an extended position, it being understood that the left side elevation view is a mirror image thereof;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a right side elevation view thereof shown in a retracted or stored position, it being understood that the left side elevation view is a mirror image thereof; and,
FIG. 4 is a perspective view shown in an extended position thereof.