What I am using for my Left 4 Dead dio-
Black foamcore board - (so the edges are black as well)
For a gritty surface-
Spray glue the surface-sprinkle playground sand on it (fine grit) Once dry, shake the excess off and re-spray with glue- this anchors the sand-
Paint with flat black spray.
Or, as mentioned above- spackle it, making sure to "flatten" it out- then spray with matte black-
For sidewalks- styrofoam - not the floral kind at Michael's, but the packing kind- I use a hot wire foam knife to cut it, score the lines in the sidewalk, etc.
Then use the grey craft paint from Walmart or Michaels to cover it, then dirty the hell outa it!
Rooftop corners, edges - Usually, there is small wooden trim pieces at Lowe's, Home Depot, etc. It's for cabinets, and is stocked by all the other larger trim pieces.