The ornamental design for the post mountable object holder, as shown and described.
FIG. 1 is a front perspective view of the post mountable object holder showing the design in an environment of use mounted to a post and holding objects;
FIG. 2 is a side perspective view of the post mountable object holder of FIG. 1 showing the design in an environment of use mounted to a post;
FIG. 3 is a top perspective view of the post mountable object holder of FIG. 1;
FIG. 4 is a bottom perspective view of the post mountable object holder of FIG. 1;
FIG. 5 is a rear perspective view of the post mountable object holder of FIG. 1; and,
FIG. 6 is a front perspective view of the post mountable object holder of FIG. 1.
The broken lines in the drawings illustrate portions of the post mountable object holder which form no part of the claimed design.