My suggestion is to use a combination of Pottery Plaster #1 and a harder
plaster like an Ultracal plaster. This makes it a bit more durable than
just Pottery Plaster by itself. Pottery Plaster#1 by itself will do just
fine though. I usually make the troughs about 1 1/2 inches to 2 inches
thick. I built a special mold out of plywood for this so I could make
multiple trough easily.
It doesn't matter if you pour it in all at once or in layers because
plaster sticks to plaster unless you are doing it with very thin layers.