How to optimise only the internal structure whilst keeping the external structure unchanged?

I am investigating a stator blade as part of a project and I am trying to optimise only the internal structure of the blade. How would I go about this? I would like the internal and external structures to be joined so that they can transfer loads during the optimisation. 

I was thinking of creating a shell and a solid component and only optimising the solid component as the 'design region' and leaving the shell as the 'non-design region.'

I would appreciate any feedback or suggestions as I have not come across any tutorials/videos with a similar scenario as mine.