I claim the ornamental design for a modular sack, as shown and described.
FIG. 1 shows a perspective view of a modular sack. The modular sack is demonstrated in a saddlebag mode. A motorcycle and rider are shown in broken lines.
FIG. 2 shows a perspective view of the modular sack in saddlebag mode.
FIG. 3 shows a perspective view of the modular sack in saddlebag mode.
FIG. 4 shows a perspective view of the modular sack. The modular sack is demonstrated in a backpack mode. A human is shown in broken lines.
FIG. 5 shows a perspective view of the modular sack in backpack mode.
FIG. 6 shows a perspective view of the modular sack in backpack mode separated from a first sub-pack and a second sub-pack.
FIG. 7 shows a perspective view of the first sub-pack of the modular sack; and,
FIG. 8 shows a perspective view of the second sub-pack of the modular sack.
The purpose of broken lines in the drawings is to show environmental structure that forms no part of the claimed design.