Do you know anyone with a lathe? It is a ten minute operation to machine one up, they will need the prop and an old Merc spacer to get the angle of the taper from. Make it slightlythicker than you need and try it, you can always remove material to move the prop closer to the box.
I have an old myford lathe and had to make my own up to fit a raker to a yam years ago.
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