@Christof
Thank you. 

 I used a single layered cardboard and tape along the edges.Then I applied wood glue and sand.When it was dry I painted it.A word of caution cardboard can fold up when drying.Add tape to the back to keep straight.You can spend a little and use plasticard if you wish and then you need not worry about it folding up. 
 @Angrist
Thank you. 

 The stones are shop made(I got them free),but I did texture them a little more.You can just make them using polystyrene,knife,hot wire,fine sand,wood glue and paint. 
