thanks all, yes foamboard for the concrete sections, i wanted to just get a feel for the initial layout first, its actually in 5 sections so i am going to add some inner wall detail and the build up some sloping to the outer edges, its actually for a chap that has asked for it specifically so i am not in too much of a rush as he has left me to just make it as best i can, so i will take a few days to look and think of my next step before i decide what detail to add (much studying of pics to be done).
having said that sandbags for the whole of the top of the walls is 99% going to happen.