Hi Tony. The only thing to be carefull with is the mix of cement / water and crush coral so the final rock is porous but not brittle. I found that if the mixture is too wet the final rock is more solid but if you want it to be porous the mixture should be like sticky damp.
The way I made mine was with plastic grid for support and plastic pipe to form shapes (also the pipe can be used for pumping water for circulation)
I also made some shapes by putting crush coral in a wood box, making it damp and shaping the form I wanted, then filling it with the mixed cement and covering it to cure.
You can also make the shape in several parts and then glue them together with same cement mixture and you dont see the joints.
Hope I confused you enough to try it...It's fun
Wetback.