After a bit of scrounging on the 'net, I find that the vast majority of axles I find as replacements will fit through the 10.5mm hole in the counter. There is one "Campy" axle that's bigger, but only one, and it's used on a Cannondale. Maybe others, but if so I didn't find them.
I have added a bit of extra "dish" to the assembly which will make the counter compatable with more of the big forks that are out there, but I don't think it is possible to make one that fits all possible configurations. It will fit the vast majority of road bikes, and I'd guess at least 70% of the mountain bikes I've looked at.
The counter may not fit some bikes with big, beefy forks, overly large hubs, disc brakes, etc. It may be that some bikes will allow it to mount on the rear dropouts as an alternate, but that isn't exactly ideal.
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I've gotten to about 80% on the final shop drawings, but ran into a small snag I'll need to address before I order. Making slow, but good progress.