We claim the ornamental design for an open top planter bag, as shown.
FIG. 1 is a front perspective view of the open top planter bag showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of portions of the open top planter bag is included for the purpose of illustrating environmental structure and forms no part of the claimed design.