I know this was an old post, but I was curious if you found anything that will work. 😕 I tried the mould making stuff you mentioned, and I found it’s really better for small, and flattish pieces.
I’m working on casting a sculpture myself, and I’m going to attempt using the Alumilite brand H3 silicone rubber for the mould, then possibly using a resin/gypsum mixture to cast it in. Not sure if the hydrocal stuff will still give off heat (even though it’s mixed with resin) once you try to paint it.. Hmm.
Anyways, if you’re still stuck, I would look over http://www.alumilite.com. Check out their how-to tutorials and stuff. the H3 silicone rubber they sell is great for deep undercuts and negative areas. 🙂