@nice

Nice box.

Looks like it’s a carcass and panels build. So the corners/feet are the four stiles of the box carcass with mitred panel grooves along their length. (The walls of the grooves would need to be mitred but not the bottoms of the grooves; they only need to be sufficiently deep, thus the stiles sufficiently thick.) Likewise the rails would have mitred joins with the stiles, plus right angle panel grooves on the bottom rails

That’s how I would try it, anyway. Not an easy one.