Hi Marcus - Yes I do that. I cast with Alumilite, using Alumilite's silicon mold material.
What I did was to take a regular 1.5" diameter casting I made and turned it close to what I wanted as a final size, leaving the through hole in the barrel smaller and the tone board section larger. That way I can bring it to the correct size easily and don't waste much material, but still get everything to fit right.
My call castings used to take about 6 or 7 ounces of material, it's a little over 3.5 ounces of now, so it's a good savings.
They are a little bit quicker to turn too, the primary shape is there already.
Happy turning - Tom