peter_4059 was kind enough to lend me some bits and pieces, including a couple of metal spacers that he machined himself. I'm getting pretty close now after quite a bit of experimentation.
The Geoptik Nikon adapter appears to be just too thick to work with my ASI1600 and EFW. In theory it should be the right size and it measures correctly, so perhaps the specs for the ASI1600 and/or EFW aren't accurate. The EFW measures correctly but I have no easy way of checking the camera back focus spec of 6.5mm (and it's not clear whether this includes an adjustment for 1/3 the thickness of the camera window and sensor cover glass).
The ZWO Nikon adapter looks workable but needs a thin spacer to get the lens distance correct. It may not be a coincidence that my adapter is actually a bit thinner than the specs say.
I also have a Canon adapter to play with if I can borrow a couple of suitable lenses.
What fun
Cheers,
Rick.