There are already a ton of these out there, but most of them were for the wrong calipers or used the wrong magnets. I would fix this, but nearly all of these don't have CAD source files uploaded in a libre format for libre CAD software.

This one might not fit your caliper (Designed for Mitutoyo and Browne and Sharpe), and it might use different magnets (8mmx3mm), but I've attached the FreeCAD source file so you can adjust it as needed for your situation.

You do have to do a quick bit of math with this tool. The actual measurement is the measured depth multiplied by 2.41. That's something that's easy to forget, so there's a version uploaded here with the equation added as a label that can be printed in a different color if you're into that sort of thing.

The throat on the standard model is 5mm and is perfectly suited for a Mitutoyo caliper, the wide model is 8.1mm and fits my Browne and Sharpe calipers.