I did make one in 2 halfs before this one actually, no idea where it is now!The diameter of the shoulder on the spindle nose. The bit that’s slightly larger than the thread, and sits snuggly in the recess in the back of the chuck. Just been out and measured mine and it’s 38.94mm or 1.53” in old money.
I like your clamp thing, but I’d have to have one that splits completely in two, as I need to use a C spanner on the spindle to remove the chuck, and a one piece clamp wouldn’t let me do that. No spindle lock you see!
@Seadog is that your one?