I printed out the T-800 exoskull form Landru http://www.thingiverse.com/thing:18317 and was left with the support material. It reminded me of burnt out buildings so I built a post skynet city. LA, NY? Could be! Now you can commemorate where you were the day the machines took over and use up your support materials at the same time!
(I used the downloaded file from Landru as Thingiverse wanted a file that is not an image. I have given ancestry to this thing but if I am out of line please let me know.)
Print out the t-800 (or other support generated file)
Keep your support materials
Krazy glue them to a piece of styrene
Spray it all matte black
Then with a grey spray
Then a light dusting of black again
When dry start white gluing on bits of green moss from a hobby shop. I also used some terracotta rubble.