I claim the ornamental design for a plastic coffin, as shown and described.
FIG. 1 is a perspective view of a plastic coffin showing my new design;
FIG. 2 is a side elevational view thereof, the elevational view of the opposing side being identical thereto;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is an end elevational view thereof, the elevational view of the opposing end being identical thereto; and,
FIG. 6 is a side elevational view showing the plastic coffin in use; the broken lines depict environmental structure for illustrative purposes only and form no part of the claimed design.