Planarity of Target Boxs using SBox

Yikes -

How do I produce target boxes for a box morph that have at least one planar side? 

My target boxes were made with SBox (Surface Box), mapped around a curved surface.  My box morph component is planar, and I am trying to retain this planarity on the exterior side. 

Would I need to first make planes for the BlendBox component?  Or is there another way?

Thanks