Gippslander:
I used shaft collars something like this: http://www.ebay.com/itm/Qty-2-1-2-Axle-Shaft-Collar-Double-Split-Black-Oxide-/361080090760?pt=LH_DefaultDomain_0&hash=item54120cf088 Of course in 2" ID size.
Something like this would also do: http://www.ebay.com/itm/Qty-1-3-4-Shaft-Collar-Single-Split-Black-/361072424796?pt=LH_DefaultDomain_0&hash=item541197f75c
The halves clamp onto the shaft, not screwed into the keyway, don't want to bugger up the keyway. I placed them a bit outboard of the Gibb key head, so one can see any movement at a glance prior to start-up, maybe 0.1" or less, but a bit of clearance.
The collars can be considered an additional safety feature, but no substitute for a properly fitted Gibb.
BTW, Jack Belk is a gunsmith, among other things, so does know how to precision fit.
Cheers,
Hugh