The last bit of joinery remaining are the stretchers between the longer front/back stretchers.  Are they really stretchers or do they have a different name?

I would have happily used the Leigh jig for this, but my older model doesn't support mortises on the face of the work, perpendicular to the edge.  So, I used the Domino instead.  I used the same mid-panel method as I did on the cabinet.

I started with some layout lines.  This one represented the location of the outside face of the stretcher.

I realized I had to shift the layout line in order to be able to hinge the short stretchers towards the center of the long stretcher.  They're easier to clamp in place when hinged that direction.

Some scrap 3/4" plywood provided a stabilizing base for the Domino shoe on the narrow stretchers.

Dry fit

With the joinery completed for the support frame, I'll do as much of the remaining shaping/sanding as possible on the individual pieces before a glue-up.  The bow sander that I just completed is a big help in smoothing these transitions.