I was thinking of making it 20mm plus nut and washer, as a matter of fact. I have no washer yet. If the size available is too large OD, I may have to machine a rounded edge on it to fit the rounded interior of the counterbore. I'll see what I have in stock.
Shorter better due to less cantilever on the crank etc.

The drawing attached is from one of my engineering/model builder/flying buddies for the hub.