Ha ! .... I think I figured it out.

lcgj, your custom made nut basically looks like the picture below. It uses the stock washer and extends into the 12mm hole reamed into the prop - for some extra thread grip. It has a shoulder that is stepped up to 1/2" OD - so that it fits snug in the stock washer. Then it has a second shoulder to form the hex (or square) nut - so you can tighten it with a wrench.

It has 10x1mm threads on the crankshaft end and 10-32 threads on the opposite end - to accept a 10-32 hex bolt - to hold the spinner cone ... yes ?

Assuming I am correct, this is a nice design. What I like about it is that no counterbore is needed. The Tru-Turn adapter for the Moki 2.10 also reaches into the prop for maximum thread grip but the prop has to be counterbored. According to Tru-Turn, the portion of the adapter nut that extends into the prop has a 13.5mm (17/32") OD (I called and asked). So this requires a second machining step before you can install the prop.

Ha ! ... I'll bet you thought I wouldn't figure it out.

Moses

edit: fixed some typos and modified the drawing - also I did not draw the neck (basically a cone shape which should be on the right hand side of the drawing - I think).