I'd suggest using popsicle sticks:
1) drag a blade screwdriver over the flat surface, to score it randomly;
2) cut it into properly sized pieces, with oddly angled cuts, maybe make them rough, or even splintered along the top/bottom edges;
3) 'paint' them with Minwax Polyshades Urethane Stain Satin Finish, using either Royal Walnut (dark, muddy brown color), or perhaps even Tudor (black);
4) if you prefer, matte coat them to dull the shine, then glue them in place.
You can add horizontal lines, small point-dents, to simulate nails, or whatever else strikes your fancy. After they are stain-varnished, these textures will show up nicely. Cheers!
