Hello all

Hopefully, one of you could give me a great advice:
Is it possible to get (without a lot of efforts) a negative image/imprint of an imported 3d-object – for example into autodesk 123d(freeware)??? Maybe I have to use another software?
I do not want to use a complicated retopology tool.

For a better understanding I try to give an example: Just imagine I want to replicate/cast an old coin. Thus, I import (from a friend) that already designed object (e.g. it has stl. format) to autodesk 123d.
-> How can I get a hollow object/coin now (wallthickness would be equal 0 first)?
Then I could extrude from that (negative, inner) surface maybe 2 millimeter outwards to finally gain the casting mold...

Hope you understand my aim(s) and first approach for a solution?

Thanks for any help!

-Mark-