If it were me, I would probably get some appropriate width Styrene strip, sand with 120 grit sandpaper to create some grain and use my NW Shortline Chopper and chop all of the bits in order to glue on a thicker Styrene sheet backer.
That way, I could paint the strips after sanding and prior to chopping to get some variation on the tones and weathered appearance, then when glued randomly, achieve the variation that I am looking for.
They look like 10-12" wide boards, assuming that the siding is 6" in width.