Didn't have the bar stock for my hub adapter (Ryobi Long shaft). Took a hole saw in drill press and very slowly cut a 1.75" hole in some VHMW plastic 1" thick, the plug (1.61") plug got dressed up and drilled out to 3/8" and tight slip fit it on the shaft, it left enough space for a spinner back plate and a 16-8 MAS prop.
16 & 18" props have a hub diameter range up to 1.5 to 1.65"+/ -.
The hub thickness ranges from .61" (16" wood) to .81" (18" composite)

Hole saw also make alum spacer plugs that can be edge dressed.
One large washer in front of prop and single nut and lock nut if available. Don't forget the Locktite!!

Lap the flywheel taper with some valve grinding compound. Give a nice tight fit to assist the keystock in holding the flywheel.

The VHMW is light (it floats but very dense.) Check out a plastics supplier for specs.