A prototype done. Intended to knock down and go back together reliably. Mortise is 1/16" deep than the tenon is long. Embedded square 1/4-20 nut to pull snuggly with the flat head bolt. Can't see the nut when the joint is tight together. The bolt head could be a cap screw and more discretely counter bored.
Not simple to draw up or quick to cut, but as I'm retired now time isn't that important and challenges are enjoyable to figure out.