I use the "party" ice cube trays to make aragacrete half-rounds (GARF recipe) then use the half-rounds glued to a styro lid to make a mold of as many plugs as I want / batch.
I make the mold with liquid latex available at hobby/ craft stores-white tub- 100% latex cut w/ ammonia.
Reinforce the mold by using layers of cheesecloth or mesh netting between layers of latex.
Good luck!-Jim
I've seen some people use quckcrete mixed with aragonite and then they pour the resulting mixture into a dixie cup to maybe a thickness of 3/8". Granted with quickcrete you will need to post-cure the plugs in tap water for a couple weeks changing the water weekly.
BTW, Steve, I'd lovet to see your tanks where you're propping your corals.
I mix about 1/4 portland cement, with 1/4 Southdown sand, 1/4 crushed coral and enough water to mix. I pour it into the top of a plastic egg crate. The part I us is just a large rectangular tray. I pour it to about 1/8" thickness. When it's starting to set (after a few hours) I take a nail and draw lines though the cement that later allow me to break the thin piece up into small rectangular or square pieces about 1-2" on a side. The square shape makes it easier to ship when I rubber band them to styrofoam pieces to float.