Well ... all done. Not very elegant (well, a bit rough round the edges tbh), but perfectly functional
In the end I found the Stanford instruction sheet only partly useful. It seemed easier just to start by taking one of the DDS and build the rest around it.
Took me about 2 hours including tea breaks and wouldn't have taken that long had I not made a basic error of measurement and had to find a way to rescue myself from it. Or had less tea maybe.
Almost the trickiest bit was making the "groove" in the backplate.
It's surprisingly rigid (although I did end up adding an extra layer of foam board as part of my rescue tactic, so that probably helps), very light, but also slightly off-balance once the DDS is in - needs a bit of extra weight at the front to sort out the moment.
The bungees to hold the DDS in tight are made from scrunchies tied together, and the pinhole from a can of Dr. Pepper someone had thrown over my garden wall.
Pinhole is ~0.57mm (measured with an enlarger), focal length is ~150mm, so ~f/260.
The next mod will be to epoxy a couple of 3/8" nuts to pieces of board, made flush with more foam board and glued to bottom and side, so I can use a tripod.
It's enormous. I can't imagine walking down the street with it under my arm ...
Here's a pic of the beast, plus the first negative I got out of it (on MGIV RC, EI6, developed in paRodinal 1:50) - scanned and inverted. I'll get round to trying to print it next week.