Before I broke down and bought a set of pin gages, I made this tool for measuring the diameter of holes too small for split-ball gages.
It consists of two parts: a housing with a lock screw and a tapered measuring gage with a cut away section that, in concert with the lock screw, prevents the measuring rod from falling out and damaging the slightly fragile tapered rod.
In use, the tool is calibrated by sliding the taper into a hole of known diameter and measuring the projection of the rod from the top of the body. With this reference point it's then just a simple bit of math to obtain the diameter of an unknown hole.
Yes, I knew full well when I made it that it could be fooled by a bell-mouthed hole. It was only used for non-critical measurements. I knew that I would eventually buy pin gages and I did.