We used old dried out live rock to make the rock wall on our tank. We do tile for a living and hubby cut it up on the tile saw. We then used silicone to glue to rock to back wall.
I have pictures of the build at this link. Of course the assembly and build are on the back pages on this blog spot.
http://dianafranks260greef.blogspot.com/
Now that the tank is almost eight months old, it looks very natural.