Hey Jim,
if you follow the thread that discussed that the other day to its end, you see that the problem arises that the 65 and 80 mm mounts that Onefinity offers do not protrude down below the Z assembly at all. Therefore they rely exclusively on long cylindrical milling motors like the Makita hand trim router or the cylindrical 80 mm spindles. This works only because Onefinity expects that you slide the milling motor more or less downwards in the mount before you clamp it (unfortunately hereby making the entire assembly more unstable and prone to chattering at high loads).
But the motors with 43 mm “Euro” mount have that diameter only where it makes sense technically: Directly where the motor shaft leaves the motor. Look at how the AMB (former Kress), Suhner, and Mafell milling motors look like: The 43 mm portion is only of a defined height, enough to be clamped at this point, and thereby – unlike with the Onefinity mounts – clamped at the technically most reasonable point, regarding the motor. So if you just create a 43 mm mount that looks like the 65 and 80 mm Onefinity mounts, and clamp a 43 mm motor into it, it is unlikely that you reach your workpiece, which usually is a few inches more below.
So what a 43 mm mount for such motors would need, is that it protrudes down below the Z assembly. How this could look like is shown in the thread linked above.
These milling motors for 43 mm mount are extremely popular in Europe and the standard in hobby and semi-professional CNC machines.
The 65 mm diameter (or 68 mm on the DeWalt) comes from hand trim routers being something you want to comfortably hold in your hand, and also to use an accessory called “trimmer base” that you can slide up and down the cylindrical part.
And hand trim routers are what Warren pointed out above: