In the meantime, I came up with what I think is a good design for a wagon vise. It's one that I can add to the thick-slab benchtop anytime and requires no hardware, just some careful chiseling.
I believe the single wooden screw with handle will provide enough pressure. I've never had to tighten the vise too hard to keep a board immobile. In fact, clamping too hard can make a board bow.
But I'd like to hear what you think. I've been known to be persuaded by wise counsel.