This shows the set-up just before pouring the Port big side. It is difficult to judge the scale from a photo, but the distance from top to base is nearly 18 inches, and the same from front to rear. The average depth is 3 inches. This is about half a cubic foot and the part weighed nearly 25 pounds of water and plaster.
The best instruction book on handling plaster I have come across is "Mold Making for Ceramics" by Donald E. Frith, ISBN - 0-8019-7359-7. He covers the history of molds, plaster and slip-making but not making multi-part moulds.
He warns of accidents if the set-up is not properly secured against the pressure of this weight of mixed plaster. In addition to wood screws at the corners, I tied the top board to the baseboard with nylon luggage straps, with two more around the circumference.

This shows the set-up for the starboard big side. All the natches are cut into the internal segments. These will locate them into the starboard big side. A little bit of the previous plaster casting of the port big side can be seen above the spine, with the natches as positives, sticking up. This area is the only bit where the two big sides touch. In this way, I hoped that any expansion would be upwards, towards the surface, and not force the existing segments apart.

I have marked with pink lines the keystone wedge. The sides of this wedge where it touches adjacent segments should be smooth, so that when the big sides are taken off, the wedge slides out. Then the inner front leg segment can be removed and then the inner back leg segment.