My personal suggestion is to speak with Pat Tafta of Tafta Inc in Donalds SC.
Pat does some low volume casting, usually willing to share his wisdom, and is a VW fanatic. (Google his name)
Pat@Taftainc.com
IIRC, the mold has to be slightly oversized in all areas due to the aluminum contracting as it cools.
The thickness of each area will affect the amount of contraction in those areas, so making the mold involves a lot of math, skill, and art. The mold is usually a large expense and must be amortized over a large volume of parts to be cost effective.