I started usIng styrofoam for troughs many, many years ago when I saw a 1997 article in the Scottish Rock Garden Club journal, The Rock Garden. As I understood the article the Styrofoam boxes' surfaces were simply roughed and painted. I made a number of such troughs and they did well. The light weight and good weathering properties made them winners. I'm sure there have been many modification since then -- mine is one of the many.