Maybe it's because my fork is the largest thing in the world but I think I calculated that I'd need 70mm to get right through and then enable it to be bolted on without it hitting the fork. If there's a chance you could make it longer, I'd almost certainly like one too.
What do you want measured? From the back of the fork to the front at the crown, ie. front of brake hole to back of brake hole? For people without brake holes though, I presume? :)
Maybe it's because my fork is the largest thing in the world but I think I calculated that I'd need 70mm to get right through and then enable it to be bolted on without it hitting the fork. If there's a chance you could make it longer, I'd almost certainly like one too.