I start in Netfabb- scale the mask to size, and slice the file without closing the objects.  I export the file to Meshmixer and select an area, then modify and "create facegroup"; duplicate and inverse the selections and delete excess file parts.

Don't forget the eye sockets!  but you can also use the meshmixer boolean option too...

Then select the group, Modify > Offset, if you're lucky then you can use the 'connected' button... if not then you can use the 'bridge' tool to connect the two shells together (then use Netfabb to close the holes {which I find to be easier than using Meshmixer)