Here is a dry fit of the legs.
It is a deviation from the plans, which called for laminating 2 boards together. But it would leave an unsightly glue line. However, I did learn, later, of another method that would not be so bad.
I mocked up with some scraps a mitered approach, but it was tricky and never really closed up. I thought of doing a lock miter joint, but just didn’t want to invest in a bit that I’ve heard has a finicky setup.
This method only involves rabbeting one edge of each piece. I then dominoed the pieces together. It seems really sturdy. My only concern is with wood movement. I want to put a plywood core down in the middle (for extra strength), but not sure how to secure it. So far, I think I can screw through the leg and into the core while attaching the knock-down hardware. And then when attaching the head/foot boards to the legs, I can domino through the leg and into the core as well.
I think this will have enough holding capability without aggravating the wood movement issue.