I need an alternative to vacform and plaster casting

[Biased]

I'm trying to find a way to replicate something using vacuum forming around the shape of the original object, and then layering some sort of resin to make a shell. The problem is that the object I want to replicate is expensive so I definitely don't want to risk damaging it by vac-forming hot plastic around it and suctioning air out and risk damaging it with any kind of pressure.

I want to find a way to make a cast to go over the original object, fill the cast with plaster for a ~1:1 model, touch up the shape a bit, and THEN vacuum form a mold over the plaster. (all this to preserve the condition of the original object.) After that I'll layer some sort of resin around the mold to make a shell. If there is some sort of material I can make the mold of without having to do the 2nd and 3rd steps I'm planning, it would be a great bonus and I would shower you with cookies and thanks.

My main question is this: What would be a good thing to make the original cast from? (Something that won't stick easily to the original surface or is known for being very gentle to plastic surfaces.)

[tom61]

If it's that rare and valuable, it might be better to just have it 3D scanned and then CNC carve a mold.

[Reaper P7]

You might be able to try using the methods that people use to create 1:1 resin molds of Warhammer 40,000 molds. Depends on size, etc.
If you don't mind my asking, what is this mystical object?