Electrical connector filing template

[rgsparber]

That template hole is a good candidate for machining on a rotary table.

For one offs, 3D printing might be good enough.

Might also be able to build the double D hole from pieces: drill hole to form rounded top and bottom. Then flank with plates to form flats. They won’t be on the same plane so filing would take some skill. If you made two and stacked them, you would get the alignment for your file.