Although I have a Chinese ripoff of the Zeroit DTI holder (in background in picture below), it eats up a lot of headroom on the mill and is a pain to set up.
I made something a bit simpler that works well for small diameter jobs and is a lot quicker to use. It consists of a steel plate with a dual diameter (3/8 & 1/2") stud that goes in my most often mounted collets. A strong Alnico magnet was fitted with an adaptor to which the DTI can be mounted.
Assembled for use, it looks like this:
It's really easy to just push the DTI to the needed offset for the part that is being centered. The magnet is strong enough that the minute pressure of indicating doesn't disturb the location of the DTI. This tool was built a few years back so I used an Alnico magnet. With the advent of modern neodymium magnets, the DTI should remain rock-solid.