I finished the monster:
I decided that I probably don't have enough rock in there, the structure is made up of almost 25 pounds of macro rock rubble. I decided on 2 strips of ceramic rock wall to add a nice 3D effect to the "ribcage" and I want to keep as much of the sand bed as clear as possible.
I bought a piece of wall and cut it in half (easy with a hand saw) and siliconed the halves to the back of the tank. MUCH better looking than what I could have done DIY with the pond foam/macro rock/egg crate/epoxy formula. The pieces weren't quite tall enough so I did make 2 small DIY rock wall spacers at the bottom part, also helped to hold the ceramics up. Once coraline takes over hopefully it will look more seamless...
more to come....