after lots of test pieces, I made the relief cuts on the crosscut sled. I found that besides making sure you're perpendicular to the grain and cutting through to the last ply, it's a lot of trial and error figuring out how many cuts you need and the spacing between them.

The cuts were filled with glue two corners at a time. The clamp is there to make fine adjustments while I made sure the bends were at right angles.

the other sides were done the same way, but with a gap between the ends. you can see I cut a half lap in the plywood and a piece of walnut. I left the walnut proud and planed it off after gluing. Instead of cutting the walnut connector to the perfect width, I left a small gap. This was to give me flexibility when it came time to square the corners. I glued it when they were square, then ran a shallow cut along the line where the walnut meets the plywood on the outside and inlayed a piece of scrap mystery hardwood the same width as the kerf.

fronts on and flush cut

you can see the inlayed kerf-width piece a little better here...and where I sanded through the veneer, hence the amber shellac...

test fit

Shallac'd