Efficient way to create a "mold" of a solid object in Meshmixer?
Surely! First of all, a mold is, simply put, the negative of the item. So we need to make such a negative and give it some thickness. I have this funnel model...



We start Meshmixer, import our funnel and then go into Meshmix and pull a cylinder into the frame. We size and align it to hide the funnel but for a tiny part at the top (or however much you need to have covered in the mold)



We click the cylinder, press SHIFT and the funnel top. In the top corner a menu pops up:



We choose "Boolean Difference", the menu changes to show this:



Next we alter the Target Edge Scale till we see the top corner (0.6 seems to work) then accept. The result is this mold: 




But that's a REALLY snug fit at this moment. 0.0 mm space. We can use that to cast our funnel... but we might want to make this a box for our funnel or we know that our casting material shrinks some 4 to 5 % when curing. So we need some space in the mold or box. So we need to shift the faces some. What happens to holes if we scale the whole thing up by... say 5 %? They become 5 % larger of course! So let's just do that! 


Edit
Transform 
hook Uniform Scaling 
pull X, Y and Z out by 1 mm (which happens to be 4 % in case of a 25 mm item.)


Use those square boxes marked with green dots to pull





Accept and... DONE! Last to do: export as STL and here you go! a Mold that is 4% larger and could be a cap for the funnel.