...I've had to waller-out the manufactured hole on all the finish washers I've used for the fit of the connector.
No doubt, SAE and metric don't play well together at all. Everything is just a tiny bit too big or small. I really wish the US would get on board with the metric system, it seems like we're the only country that still uses Imperial measurements that's not still stuck in the 3rd world.
Washers are very hard on grinding bits and files since most are SS steel. They are very functional though. I got to where I used nickels, they don't destroy a bit instantly (I can do 3-4 with one aluminum oxide bit before I notice significant degradation. Diamond bits would be better but are more expensive) and are thick enough to make concave like a finishing washer.
Still not an answer to your question per se, but I went this route because I couldn't find drip cups either...