This is a simple caliper for measuring the internal width or diameter of an object. I designed it from scratch but based it off the design of sulleyneal's finger calipers (which were themselves adapted from Ian White's finger calipers).

You can use layered color changing to achieve the white painting (triangular indicator on the non-ruler piece and hashes and numbers on the ruler piece). Just switch to white for the three layers (or more) beneath the painted areas and then switch back to your base color the rest of the print. The .3mf files include both colored and non-colored plates.

Print with auto tree supports enabled and no supports on the model.