Step 1: Modeling in Rhino
First create an oval for the middle of the rib and then draw an outline for the shape of the rib.
Next copy and paste two of the oval shapes and move one up 1/4 of an inch and one down 1/4 of an inch.
Now loft your top oval to the rib outline, then the same for the bottom outline.
Once the shape is lofted, loft the top and bottom ovals to close the shape.
From here turn the shape into a mesh using the mesh command.