I agree with George. If you PM me, I can send you video of how the fiat clutch fits. The Fiat clutch has 20 splines, but it as a 30 degree angle as opposed to a 20 degree angle on the Leaf motor.
@GeorgeC - it seems these files are at: For Sale: em57 leaf motor couplersA drawing would be useful, as my other thoughts were to either fabricate it out of 5mm aluminium or carbon fibre, however having a motor side gearbox housing gets me most of the way there - I would only need a piece of 5mm aluminium sheet to mate up to the transaxle.
I bought a Fiat124 clutch on the strength of what others have said, but I don't think it's a very good fit - there is some play in the splines, which I think will just wear the motor shaft splines down in time. The Leaf primary spline/gear is a much better fit and has the benefit of an intermediate bearing.