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You can also bring the model and a big box into slic3r, align and orient them (enclose the model in the box), and do a subtract modifier, leaving a hollow where the two intersected. You probably want to do this twice, for a top mould and a botom mould. I've done this, but I don't see any instructions online for it. :(
EDIT: Unfortunately, this would be very tedious. It's much easier to use meshmixer or another publicly available program to subtract one stl from another.
In Slic3r, using another stl as a modifier has no effect unless you are also printing that second stl (normally from another head). So you would have to manually remove all the gcode for the second head. Sorry for the bad advice.