I claim the ornamental design for the plastic bag box holder, as shown and described.
FIG. 1 is a front view of the plastic bag box holder.
FIG. 2 is a right view of the claimed design.
FIG. 3 is a side view of the claimed design.
FIG. 4 is a top view of the claimed design.
FIG. 5 is a bottom view of the claimed design; and,
FIG. 6 is an isometric view of the claimed design in use with and supporting a box containing a roll of plastic bags and of which the roll of plastic bags exit through an opening on the front of the claimed design; and wherein both the box and the plastic bags are depicted in dashed lines forming no part of the claimed design.