It sounds like a great idea and I'm going to try it on my next tank to build large caves for my angel fish and tangs.
Quoted from the thread:
The owner explained to me how he made them.
1) The pvc racks were constructed.
2) Styrofoam in a can was sprayed onto the racks. I didn't ask what "styrofoam in a can" was but I think it was that stuff you use for sealing windows and doors.
3) Fiberglass was placed on top of the foam. I asked what the purpose of this was and he said it helps the stuff in step 4 stick.
4)Rubble from sand was literally thrown at the foam to cover it.
The finished result is a pvc rack that looks exactly like live rock.
Here is where they start talking about it
http://reefcentral.com/forums/showthread.php?p=5977082#post5977082
And here is where the owner describes it:
http://reefcentral.com/forums/showthread.php?p=6000997#post6000997
I guess they float because of the styrofoam, for a glass tank you could silicone them down. Also use pvc flanges to distribute the weight on the bottom of the tank.
I would recommend that thread for anyone, it has lots of great information.
Sorry to hijack. If anyone has more questions, I will open a new thread.