If you're getting a box in a box then you are probably simply shelling and not strutting.
Take the outer box and boolean it in half - if it's hollow then you are shelling it all right.
Try the following:
- Start a new scene
- Open the Shell properties dialogue and set as shown in first image below
- Create a primitive cube
- Right click the cube to enter point edit mode
- Select all faces of the cube (I use 'select using rectangle' with backside enabled)
- Shell the cube
- Right click in workspace to exit point edit mode
Result? I get second image shown below
By selecting only certain faces I can selectively strut as shown in 3rd image below. Note the cube has had extra vertices added to show the effect.