dhb, the files came from one of the common part sharing site. Not, thingiverse, if I recall correctly.
100% for 29mm, 82% for 24mm, 61% for 18mm. Trying 45% next
They take about 20 minutes of finishing with needle files to get to thread smoothly.
I don't have the solidworks skills to try making a matching staging adapter - but I did think of it, too. I'm going to take examples to my coworker who's printing them for me, and see if his boy (grown, supposedly skilled in the art of solidworks) can make matching staging nuts.