Hi Stuart - it looks like both Jamie and Burr have you covered with some steps to get what you want above?

That method of trimming out a piece and then either shelling or extruding the trimmed fragment is probably the best way to do this.

Also check out these previous posts from some other illustrations on this method:

http://moi3d.com/forum/index.php?webtag=MOI&msg=3024.4
http://moi3d.com/forum/index.php?webtag=MOI&msg=3318.1

- Michael