I made a magnetic base for a plunger clock to slap on to any tool holder, I dont have many tool holders to dedicate one for this. I have three of these shop floor DTIs that were thrown out at work after they were gummed up and dirty. Our inspector was told to look at them and could not be bothered (not his job apparently) so I asked if I could have them for spare parts and he said help yourself, they lost the backplates but no problem to make and the screws were 8BA which I already had in stock.
After a proper alcohol cleaning they worked fine so I made a backplate to take standard clamp which will work for diameters and faces and takes 6 neodymium magnets. This might slightly magnetize the tools but I have a demagnetizer to take care of that. I usually use a TESATAST finger type indicator but there are times when plunger ones are handy for clocking square stock and offsets etc.