use the shell command under modify - Solids editing - shell
or put a shell tool on your toolbar (found under customise modify)
select the object to shell
(object highlights and command line changes to)
Remove faces or [Undo/Add/ALL]:
If you want the object to have open ends/sides, select the faces that you want open by clicking on an edge.
This selects 2 adjacent faces (see command line echo) click the face you don't want whilst holding the shift key to de-sellect it. (use a different edge to the one you first selected)
repeat for any other open ends (don't forget to de-sellect unwanted faces)
ENTER or Right click
input shell thickness
ENTER
Job done
Regards Fozzie