It is a frame ... but a standalone "frame" viewed from all sides, similar to a sheet-music stand. I would consider bridle joints for the ends, but I'm less enthusiastic for parts that join in a "T" fashion where the bridle really has to go on the outside of the rail (top of the T). I'm really looking for a production tenon cutting tool ... and ideally one that already has tenons available (Domino) or that can be easily fabricated.
There are lots of great ideas posted here and I appreciate all the thoughts. Today after a few chores, I am hoping to get out to the shop and experiment. First thing I would like to try is to setup the Domino like the referenced post. That seems like a versatile setup. I also reached out to John about the HRM which also looks exceptionally versatile.