Adding a reinforcing rib to STL model with blender I'm wondering about how to add a reinforcing rib to a stl model with Blender.
I have a shape like the one in the picture below and  I'd like to add a rib that perfectly match with the inner surface which is curve.

In a CAD modeler such as Inventor there is the possibility to extrude a sketch to a face, but no idea about how to do it in Blender.

I'm able to add a cube and resize it to the approximate shape of the rib, but how to make its border to match with the inner surface of the original stl?





Thanks in advance for help!
In this particular case, I think it can also be done in this way:

put the original mesh in front view, wireframe shading,in edit mode, 
then select just the central array of vertices, 



then duplicate (shift-D) and separate (P => "selection") those vertices (to a new mesh)

as you see in side view, you now have a "section" object made of two "loops".



deleting the "inner" vertices from both loops, you can then make a single "face" joining the remaining vertices on their edges in pairs (upper and lower) using F then, after selecting all of them, make it a big "face" pressing F



now you can extrude this face on left and right (I would not bother using mirroring), obtaining a solid central shape that perfectly follows you original mesh curvature. 



Then I would join both objects, and export to .stl again.