How do I create a solid envelope from my assembly?

I need to create an shrinkwrap with just the outter surface to create a single "solid with no voids" part file.  When I use the shrinkwrap with MERGE-SOLID it is still showing all the void areas inside my assembly.   What I need is sort of like a facet model but using the outter surfaces of my assembly to genertae the part.

Using merge solid option

Using Facet option