Thanks to BillH! An example of how useful this forum is! The first thing I made for my JET 9x20 three years ago was a carriage stop. I used Delrin because it is easy to machine, seemed to be strong enough and wouldn't scratch the ways. It never occurred to me what a handy indicator holder it would make. Every time I needed to measure carriage motion I rigged up some awkward arrangement to hold the indicator. The photos (if I've managed to post them correctly) show the original stop and, thanks to a word from BillH, its improved form.
And for the picture posting technique, thanks to Evan (and a friend who pointed out to me that my ISP probably provided me with web space at no extra cost.)
Although friction seemed adequate to keep the indicator in place against gravity and vibration, I grooved the 5/16" indicator stud and put a plunge ball in the side of the Delrin block for security.
A few more pictures showing the details of the indicator socket can be seen at