Hi Oliver - shell in MoI for thickening a surface can only really work on a single surface or one that is all smooth. Areas that have several surfaces meeting at an angle to on another are much more difficult because the offsets become separated and need to be extended out in order to meet up with each other.

If you make your object a closed solid rather than a joined surface by capping it off, shell might have a better chance, but what I'd probably do would be to manually make the inner core by doing a curve offset of the top and bottom pieces and then lofting between those, then do a boolean to cut it out, see attached model.

- Michael