It is important to get the holes in the washers and in the field-frames aligned properly. The easiest way to do this is to create an accurate template for marking and/or drilling the holes. This article describes one way of making such an template from steel. There is another article that describes the manufacture of a plywood template.
The process step by step Edit
First make a CAD model of the washer's rim with a sufficient number of holes. Next glue it to a piece of thickish (3-4mm) steel:
Mark the places of the holes with a punch:
Drill the holes on the rim and in the middle. The purpose of the middle holes is to remove as much material as possible:
Remove the piece of steel in the center:
File the center round:
Expand the center hole until the washer core fits snugly into it: