Thanks Splintergroup!  
I did set the box to the side of the blade, but somehow misjudged so the cuts were too shallow. Guess I was being overly cautious on not cutting into the box.  The second box went much better so hopefully I'm getting better at the setup.   I did follow your comment on my prior project of waiting to cut the top free until I had all the splines done.  Given the sides were much narrower, this allowed for all the corners to be "set" before separating the parts.  Appreciate your help!