It used to be pretty common to make reef rocks. You can make all sorts of custom shapes. To make it puorus add rock salt to the cement. It will disolve out when soaking to decrease the PH. People used to but it in their toilet tanks until the PH drops.
There was a company that had lots of info on making your own rocks on their website sites. I can not think of the name at the moment. Search the old Reef Central archive threads and you should find lots of threads on making rocks.
Personally, I would never set up a tank without a good portion being fully curred live rock.
I have all the materials to make rock I bought years ago but never used, if someone is interested.