[MR] Metalcasting from a positive master - expertise needed

[Gorm of Berra]

Okay, I *KNOW* someone out there knows the answers to the questions
I'm having regarding making pewter casting molds from a positive
master.

Sandcasting is impractical, because I need 600 copies (that'd take forever).

I don't really want to do soapstone carving, because a) the Master has
lots of very find details and my stone carving isn't that good yet and
b) soapstone masters have lifespans of around 100 castings, and I need
600, and 6 identical carvings would, again, be a...stretch...of my
skill level.

So, I recall from a class I took once upon an event years ago about
using Bondo to make molds...it worked in the class, but obviously my
notes are missing some critical step, because my mold just isn't
working out.

Can someone out there with expertise kindly assist?  thanks!

Gorm