After trying (successfully) to make a radius gauge that measured the diagonal, I found it was too time consuming to make your calculation after each measurement. This one requires a little more careful measuring but makes use of the fact that flat depth gauges on verniers are centered to the caliper body. Extend the depth gauge until it starts to touch the object, take your reading - the gauge should be accurate from 0 so no need to rezero. This will slip on to most brands of calipers, it is not a tight fit but will sit square. Obviously, only useful for right angles radii. Old radius gauge for measuring the diagonal shown in last photo - not particularly convenient to use so I'll only upload it if someone wants it.This one printed with 20% infill, 0.3mm layer height, no supports. You may or may not need supports depending on how well you can bridge.