Creating 3d printed stencils from a 3d model


First of all, hello all. I tried doing some searching but failed to find what I am looking for.

Here is what I want: I want to be able to take a 3d model and build a shell around it so I can print the shell with certain areas exposed (ex: frame work of a canopy so I can paint the windows on a model with an airbrush, leaving the fame unpainted). To try to be clear I will break it down further:

So I would

  1. use a scanned 3d image of the model (lets say a model stormtrooper)
  2. buildup over top of the model (a mm or 2) and be able to remove areas from this outer shell to then 3d print the stencil and paint the open areas
  3. build several different stencils so I can have one for uniform area, another for the armor etc and maybe one for the weapon

I have zero budget so will have to use software that is available free

I am completely new, sorry for that and thanks in advance.