Managed to get two sets of parts cut:
Which shows more or less how it ought to work.
Not included in the photo are three additional boards (all four would be cut at the same time) and a front cross bar which the two F-clamps would hold in place to clamp the four boards, and a set of adjustable clamps which would hold the boards from side-to-side (still need to cut those, but not convinced of the design yet).
The brass nuts are speed nuts from:
https://www.garrettwade.com/frame-clamp.html
and ideally I’d have a bunch more of them.
A nicer version is available from:
http://www.leevalley.com/us/wood/page.aspx?p=31162&cat=1,43293
(but of course, I just missed free shipping, story of my life)
cncfingerjointbox_fixture_secondary.c2d (14.7 KB)