re: Wood movement for a box

Oh, I was thinking that the space balls would have helped avoid the corners sliding inward given the panel was intentionally undersized.  

Is my method of just gluing the center of the panel a good one?  I didn't put any glue along the long edge.  Other suggestions?  

Wish there was a way to look inside the dado to check on spacing!
 
Will sleep on it and see how I feel in the morning.