The ornamental design for a rainwaterfiltercollector for a downspout, as shown and described.
FIG. 1 is a left, front perspective view of a rain water filter collector for a downspout, the right, front perspective view being a mirror image thereof;
FIG. 2 is a right, rear perspective view, the left, rear perspective view being a mirror image thereof;
FIG. 3 is a left side view, the right side view being a mirror image thereof; and,
FIG. 4 is a top, right perspective view, the top, left perspective view being a mirror image thereof.