Slightly OT chaps as I know you're looking for washers for 20 mil mini's so I do offer you my sincerest apologies in advance
I use 2 pence pieces (for those outside the UK they're approximately 25mm in diameter. I always place the queens head on the bottom of the coin as I believe it's still an offence to dengrate a picture of the queen!! It's the 20th century and all that too!
coming to think of it, Vivian Westwood got away with it so why can't we?
Darrell.