This is by no means the only way to do it, but it is works pretty well for me. I start by building a base between the legs out of clay, this will be the third section of the flexible mold. I turn the whole thing on it's side till it's parallel to the board and support it with a wad of clay.
Next I measure from the center line of the model to the board and use that measurement to cut blocks of clay at just the right height and wedge them up to the model to create the parting line for the mold.
Here you can see I fill in and smooth out the clay.
Next I use foam-core board and build the sides, (the reservoir for the liquid rubber), making sure everything is squared up nicely. I use tape to hold it all together. I tried using a brush handle to make the holes (Keys) for locking the mold together but I found that using ball bearings works better for the first pour (less trapped bubbles).
After the silicone cures I flip the whole thing over. This photo is not from the same project but you can see what it looks like from the other side.
Here's what it looks like when the clay is all cleared out. The beauty of using the foam core is you can pop it off and clean out the clay and it fits right back on. This is a shot of another project. The wood blocks are there so I won't have to use so much silicone and also to support the mold in the roto-cast machine. I used ball bearings on this project. It's much cleaner than poking it with a brush handle.
Here's a shot of the first two sides completely cured and the base cleaned out and ready for the final section of the mold.
With the pour completely cured I go an extra step before taking it apart. I make a box (in this case thin plywood that I had painted on previously) around the mold that will act as a shell to keep all the parts together while in the roto-cast machine.
Here's a look inside with the model removed. We're ready to pour our casting material. The base section of the mold acts as a pour hole. Any seams will be between and underneath the legs.
Casting material poured. It's on to the roto-cast machine. Don't laugh it works!