Offsetting a solid part with complex geometry (creating inner core of an existing model)

I am looking for a solution on how to model inner core of an existing model. With the model I have already produced the product. For production there were used two molds. The production method is RTM technology (epoxy resin injection technology with glass or carbon fiber).

Product:

Computer molds (negatives of the production molds):

Production molds:

Molds with the offset-ed surfaces 1, 1,5 and 2 mm to the inside of the model for production of the polyurethane core (offsets were made by hand in this case):

Polyurethane core (from the offset-ed molds) is later inserted, with the glass or carbon fabrics, in the production molds and resin is injected. This way the final product is made.

Problem is that these offsets are made by hand with offset wax and are not so precise, which affects the performance and weight of the final product.

I am trying to model the offsets to have them CNC produced. In this way I could produce a better product. When I am offsetting surfaces from the existing geometry of the computer model I created, I am encountering numerous problems with offset surface, thickened cut.. a lot of errors when creating new geometry, because of intersecting surfaces,.. etc. .. 

Anybody has a good idea how to approach/solve this problem?

BR,

Matic

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