My spindle is 20mm bore, and I had some 20mm PVC rod, so I drilled it through, and put a taper in one end. With a long bolt into a matching cone nut, it expands to a good grip even without slits.
Peter, I try to remember to unplug the lathe when I'm hand cranking, to stop myself starting the motor and suffering whacks on the elbow from the crank handle as the lathe attempts to turn somersaults. For years, I've been meaning to substitute a wheel for the crank to introduce a bit of balance; something like 6 - 8" diameter, maybe off a child's discarded ride-on toy with a rubber or moulded plastic tyre for grip.
Andy