I claim the ornamental design for a block for a garden box, as shown and described.
FIG. 1 is an isometric view of a block for a garden box of undetermined length and having a repeating pattern along the length, the garden box including my new design.
FIG. 2 is a top plan view of the block for a garden box.
FIG. 3 is a front view of the block for a garden box, the back view being a mirror image of the front view; and,
FIG. 4 is a first side view of the block for a garden box, the second side view being a mirror image of the first side view.
The broken away portions shown in broken lines represent indeterminate length. The alternate dash-dot lines represent the bounds of the claim, and all other broken lines are included to for the purpose of illustrating the unclaimed portions of the block. None of the broken lines form a part of the claimed design.