Maybe I can help a bit here.
If by 'shank' you meant the 8 sided drive gear, the actual size is 15/16" measured across point to point, or 5/8" measured along the sides. An odd size I know, but one that was standardized in the industry in the early 70's.
The length of the 'shank' varies depending on brand of manufacture. Not all winches are created equal - so too with winch handles. Not all of them will fit all winches. This is especially true with locking winch handles.
In any case, having personally gone through the 'let's build a winch handle' challenge I can say that it is far more involved an undertaking than you might expect. Issues of fit, strength, form, and function all must be gotten right to end up with a solid piece of gear. I would not recommend undertaking such a project in your home shop.
Will do my best in responding to any other questions you might have on the topic.
Regards,
Don Steiner