Thanks, and happy sneaking.
~the art ninja

In the two side profile shots you can see the seam line on the figure. It's actually fairly thin, with a small amount of flash showing. (flash = thin amounts of a casting's material that has seeped into the parting seam between the two mold halves. )
In this shot you'll see the first half mold support structure which I built from sheets of craft foam. The back is built up using a hotglue gun, and slowly but surely, pieces of foam have been added to completely isolate one half of the sculpture. I've placed glue pips on the side which will create keys to hold the mold halves together better. You'll see what I mean later on. In order to completely seal the parting line edge to prevent the mold material from leaking through to the back, when the support walls are complete I then carefully painted a layer of vaseline along the edge to plug up all the holes.
I've added the two side walls, Only the top wall to be attached then pouring starts below...
After the support structure is built to isolate half of the sculpture, I mix up a batch of RTV silicone. Catalyst is poured into the liquid RTV which will cause the silicone to set within 24 hours. Mixing introduces lots of air into the silicone as you can see from the image. After it is mixed thoroughly...

I poured the first half of the mold in 2 stages. This first pour was done this way so that I could get a thin layer of silicone to seal any gaps which I might have missed along the edge of the figure. Even so, I discovered after that there was in-fact a thin gap through which silicone leaked. If I had poured the entire side at once, I would have lost a lot of silicone through that gap. The mold was left to set for 24 hours at this stage.
This shows the second stage of the First pour. In order to avoid trapping air against the various surfaces of the sculpture, I pour the silicon into one corner of the foam "box" and allow it to VERY SLOWLY flow over the surface of the sculpture. If I was to carelessly just dump the whole batch of RTV into the box it would trap air which I do not want to happen.
The First pour almost finished. The entire sculpture is now covered. I added more silicone after this shot to make sure that the final mold would be thick enough to hold its shape when casting.
After the silicone sets for 24 hours I pull off the finely crafted foam "box" taking care not to damage the sculpture or cause the RTV to come away from the figure. I need it to stay exactly where it is in order to build the second half of the mold.
Here's the cleaned half mold...You can see the divots created by the hotglue pips added at the beginning.
Fast forward a bit...I've built the walls for the second half of the mold. Reinforced with cardboard to keep it from coming away from the completed sides.
Here's the final pour. Time to wait another 24 hours...
After it's set, the box comes a way easily. We're left with a solid block with parting line along the side.
The moment of truth. The mold opened. The sides came away easily (because of the vaseline used as mold release) Looks clean, no bubbles and the sculpture looks to be intact!
Carefully I pulled the front half of the mold off the sculpture. The silicone is extremely flexible and is great for casting sculptures with undercuts. It comes away without much complaining.
Here it is, the final shot. Clean mold ready for casting and an undamaged clay sculpture ready to be used to make a new mold if need be!