Here is one tip...
One thing to be wary of when casting pieces which will fit together
is the shrinkage factor. A piece that is much thicker than another
will shrink more, and perhaps unevenly, so compensation may be
required. As an example, back in about 1975 a jeweler approached me
to mold several pieces to be cast into gold and fit together. They
had different thickness, and as a result, were not a close enough fit
to be used. In that case, there were 3 shrinkage factors, the mold,
the wax, and the cast metal. As a general rule, pieces of the same
wall thickness will shrink similarly.
Personally, since I do cad/cam work now, I usually mill wax patterns
which must fit each other, or use Rapid Prototyping. I am usually
able to achieve a close fit.