Inspired by the model of the same concept as shown here on printables: https://www.printables.com/model/236582-radius-caliper-no-computation-required I wanted to rectify one major downside to the original design, being that the acute angle doesn't allow you to measure radiused corners greater than 60 degrees.  This adapter uses the same math to provide a direct radius measurement but has one major advantage.  You can screw in M3 socket head screws to act as a stud.  This enable you to measure radiused corners that before were not possible.  At the same time, there's no conversion or math to determine the radius.  The screen shows the exact radius (plus or minus margin of error of course).  I've also included a small window so that you can ensure your caliper is seated all the way.  The caliper I used is a model from Husky, they are inexpensive at $25ish dollars.  It's very likely that your calipers will fit as well.  I've left no tolerances because side to side runout will affect accuracy.  So you'll need to press the calipers into position the first time you use this.

To get proper measurements, it is important to make sure that the radius does not exceed the tangent line of the adapter.  For example, if you're measuring a corner, position the M3 studs in equal holes that place them at a rough equivalent of the radius you are measuring.  If the radius were to exceed the triangular tangent lines, you need to readjust the studs.

A more accurate measurement will result from minimizing the area of the circle that would exceed the tangent lines of the tool.  Please refer to the pictures for examples of improper vs proper usage.