I made this fixture by ripping a 3/4 wide strip of Baltic to define the width of the slot, then reglueing it together. I used a 3/4 collar and a 5/8 bit.
In addition to mortising for the hook to enter the leg, one also must mortise below that point for the hook to move toward the floor for complete engagement. I discovered that laying the plate in one way, marking, then the other way, and marking again, made a perfect outline of what needed to be removed. I used the drill press.
-- "...in his brain, which is as dry as the remainder biscuit after a voyage, he hath strange places cramm'd with observation, the which he vents in mangled forms." --Shakespeare, "As You Like It"