Create panel > Geometry > Extended Primitives > Object Type rollout > C-Ext button
Create menu > Extended Primitives > C-Extrusion
Use C-Ext to create an extruded C-shaped object.
To create a C-Ext object:
From the Create menu, choose Extended Primitives > C-Extrusion.
Drag the mouse to define the base. (Press Ctrl to constrain the base to a square.)
Release the mouse and move it vertically to define the height of the C-extrusion.
Click, and then move the mouse vertically to define the thickness or width of the walls of the C-extrusion.
Corners—Draws the object from corner to corner. You can change the center location by moving the mouse.

Back/Side/Front Length—Specify the length of each of the three sides.
Back/Side/Front Width—Specify the width of each of the three sides.
Height—Specifies the overall height of the of the object.
Back/Side/Front Segs—Specify the number of segments for a specific side of the object.
Note: The object's dimensions (Back, Side, Front) are labeled as though it were created in the Top or Perspective viewports, and seen from the front in world space.
Width/Height Segs—Set these to specify the number of segments for the overall width and height of the object.
Generate Mapping Coords—Sets up the required coordinates for applying mapped materials to the object. Default=on.
Real-World Map Size—Controls the scaling method used for texture mapped materials that are applied to the object. The scaling values are controlled by the Use Real-World Scale settings found in the applied material's Coordinates rollout. Default=off.