I know there have been numerous threads on building rock, but I think I make it differently. I make it hollow so besides allowing me to make it any shape, it also helps grow anerobic bacteria that will help with nitrates.
My entire reef structure is supported on this type of "rock" and I can build rocks very long and thin, something that is not easy to build with other methods. It is also very cheap, practically free and only takes about 15 minutes over 3 or 4 days.
First I bend a piece of PVC pipe. "This MUST be done outside with the wind going away from you." Melted PVC fumes are toxic and will sicken you instantly. If you use a heat gun it is better as it will not char the PVC but I use a propane torch and it must be done outside. You do not want to sniff any melted PVC. Trust me.
I play the torch over the pipe until it softens, then I bend it in odd shapes and stick it in water. You need a hose near by because sometimes it may go on fire. If you keep the torch a distance away that will not happen but keep a hose ready anyway.
I use "Sakrete" cement.


Then I cut the bent PVC in the shapes I want it. These smaller pieces will be used to support my reef structure when I re aquascape shortly. The larger piece will be used to support a coral.


I attach a base of acrylic plastic. I use a stainless steel screw and thread it into the acrylic, then glue gun the head of the screw into the PVC. Some people are afraid of stainless steel so you can use nylon screws. I have holes drilled all over the PVC to facilitate water entry. When I stick the head of the screw into the PVC pipe, I just stick the glue gun into t lower hole and fill it with hot melt glue.
String is wrapped around the PVC and secured with hot melt glue. Then cement is mixed kind of loose, this is smeared all over the string (wet the string first, the cement will stick better)
You will get a thin coat of cement all over the thing, but you can not finish it all in one day.
Put a bucket over it and let it set for at least two days. Then blob on more cement and wait sanother 2 days. Neatness does not count and you want the cement blobbed on rather than smoothing it on. Eventually, you will have something that will look better than any rock you can buy and coraling algae grows much better on cement than it does on real rock. Cure it in some water for a few weeks before using in your tank.
Here I started to put cement on one piece. The shell on top will eventually hold a coral.


This piece (before and during construction is 3' long)



Here is another piece supporting my structure.


This "old" bottle was also constructed using cement.


Thanks, you can see one of those skinny rocks here above the copperband

You can see the larger rock here behind the moorish Idol and all the way to the left under the torch coral.



Here is a piece that was in my tank for a decade or so


It kind of disappears in my tank