You can do that by using Segment Editor module (before FEM mesh generation). You can grow a shell on the skin using “Hollow” effect, then cut out rectangular shapes using “Scissors” effect.
==> good but, in the aspect of user, like fiducial tools(below picture),
based on user fiducial 3d point on the surface mesh, it is good to draw for the attaching cube (below picture by the hand) which follows surface normal vector
You can hide the background mesh in Models module / Display / Scalars section, enable Threshold and set lower value to 1.
==>I think it is homogenous mesh than tetzen. i am now tunning cleaver parameter^^