Yep, Bill beat me too it, sliding dovetail would be a very nice solution, the rail would be hidden behind the solid wood panel, and the rail would have the "Male" part of the dovetail, while the panel would have the "Female" part, or groove in it, make it a bit loose, (only a bit) and put a dab of glue in the center to keep it from rattling. The legs would have a regular stopped groove on them to pick up the tenon on the three sides of the panel.
This would have a very nice clean look, and even if you flipped the unit upside down and took a look, it would still look clean and simple, with no obvious bolts or screws showing, elegant and simple.
The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.
William Arthur Ward