Use the LINE command to connect the end points of the entities at both ends thus completing the angle section in to an enclosed " L " shape.
Use CHPROP, T to assign a thickness of +175 ( 7 inches) to each of the six entities. This is similar to Task 4 earlier, and the entities need to extend away from you toward the ucs icon shown in Fig Tsk 23. (A negative value would force the thickness in the opposite direction away from the ucs icon.)
Thaw / turn on layer 01 by clicking on the snowflake icon after typing ‘layer.’ (For older software type ‘layer > t > 01’ followed by 2 returns.)
Erase the cross from the top of the box.
Copy the circle on the front face of the box to the back face. This can be done by using the corners of the box when given the "base point" & "second point" prompts. Choose a front corner followed by the corresponding rear corner. Alternatively, copying by @0,-3,0 (ISO = 0,-75,0) will achieve the same result.
Please feel welcome to make use of the free resources at the side and bottom of these webpages. Many of them contain very useful 3D CAD material.