The ornamental design for an attachablewriting implement holder, as shown and described.
FIG. 1 is a perspective view of the attachable writing implement holder, illustrating my new design;
FIG. 2 is a side elevational view of the attachable writing implement holder;
FIG. 3 is a top plan view of the attachable writing implement holder;
FIG. 4 is a bottom plan view of the attachable writing implement holder;
FIG. 5 is a front elevational view of the attachable writing implement holder; and,
FIG. 6 is a rear elevational view of the attachable writing implement holder.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design, while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.