A lot depends on how you're going to join the sides and ends together. Pictures show a simple butt joint, is that accurate? Do you plan on screwing it together then plugging it or.......?
Were it me, I'd make a vertical rabbet on both vertical sides of each short end. About 1/2 " deep and as wide as your material is
( 3/4"?) So, long sides would fit into a corresponding rabbet on the ends. This would allow you to dado in for the bottom, as long as the dado was shy of 1/2"deep no gap will be seen. Also, it makes the corners look reasonable, in that the corners would look close to the same width from any direction, if you planned it well they would be identical on 4 sides.
Glue up is a bit difficult with this method, lots of clamps or a nail gun, but it does allow for a decent dado for a ply bottom. This is confusing for me just to write, hope you can make sense of it.