There a lot of pretty nice, cheap magnetic arms available that were designed for holding dial indicators. This clip is a replacement for the dial indicator clamp part, that allows the arm to be used to hold 

You'll need:

• 7x19mm compression spring (or similar)
• Small nail
• 1pc ‘base’ part
• 1pc ‘clip’ part
• dial indicator stand (I paid €19 for mine)

Use the nail and a hammer to lightly push out the pin holding in the original end holder. Swap in the ‘base’ part, then use the nail as a hinge to hold the ‘base’ and ‘clip’ parts together. Compress the spring, and place it over the mounting points between the base and clip.

Note that the quality really depends on the string strength. I designed the clip with 7-17mm diameter probes in mind, but this is a wide range for a small spring, and the pressure isn't as good enough as I would have liked. But not bad for €19 and some junk parts!