Jim's right,
A rabbet joint is fast, simple..and with good glue and plywood , pretty strong. I would use a 1/4" dado for the cuts, rather than repeated saw kerf cuts.
It's even stronger if you dado a groove around the bottom to take the bottom piece..probably 1/4" ply, and since it's plywood you can glue the bottom piece all the way around in the slot. Pay attention to your assembly process..do a dry run first.
Don