I haven't done it yet, but I am currently gathering the stuff to do it. I am planning on going with a 4 part aggregate to 1 part cement recipe, likely doing 1 part pulverized lime:1 part sand: 2 parts crushed oysters: 1 part portland cement. I'm planning on mixing up a few batches to experiment. I have the sand, cement, and lime currently, and I think I've got enough to make about 100 pounds of rock for less than $12 so far. I expect to spend about $16 on the oyster, another $10 on sand for molding and plan to use the big flat foam fish boxes to hold it all. I've got to get another trashcan and a power head to cure it all, I'm going to hook the effluent from my RODI unit to the can to get regular water changes into the can. I'm working on some different ideas, but I've been spending a LOT of time thinking about the shapes I'm going to do, and I believe that by going with flatter pieces and using a slightly sharpened dowel rods to poke small holes in the molding sand I can create tabling acro sort of rocks, similar to the EcoRox or MarcoRocks of the same shape. That's the goal anyway, since I'm going to do more of an open tank with just a few ledges for corals.
That being said, I'll spend less money on the new 90s rocks than on all of the other tanks combined, and I hope to make between 150-200 pounds total. I only expect to put MAYBE 100 pounds in the display, and the experimental pieces will be used for the sump as rubble. Out of that amount, I also plan on maybe 20-30 pounds for each of the other 2 tanks.
I'll definitely be documenting my process, and hope to start after Christmas once I gather supplies. I'm not sure how weather affects curing and stuff though, so I may have to wait.