I made three sets of classroom mailboxes, for passing in papers / storing journals etc. Sides and back are 3/4" plywood, shelves are 1/4" plywood. Corners are rabbet and dado joints, my first time doing that. I did the cuts on my table saw. (I tried to route them, but didn't get as clean a cut as I'd like with the cheap Ryobi bits I had.) Shelves slide into dados. The sides/centeres are designed so one fence location could cut the top and bottom dado. I didn't have a dado stack, and am using a Shopsmith which has the table saw blade arbor on a quill, so I set the quill stop for my dado width and used that to make multiple cuts slightly apart. That worked fairly well but must have been slightly off on some cuts where it was very hard to slide the 1/4" plywood shelves in; I ended up sanding the edges of some slightly thinner.
