You could always make a female cut in a template, minding the width at the base for the blade itself. Once your to the back of the blade, the rest isn't so critical. Cut small, and widen it up till the blade clears in all of its rotation..
If your not in a big hurry, and want to spend a few bucks, :)
I'll have my mold done soon for a 12" screw-less spinner kit, all carbon, including forward and rear bulkheads. My prototype part made it through a week of Reno Air Racing turning 3000 RPM and exceeded Vne. So I know it works. Send me an email if you want to see some pics of the parts.
Tom McNerney
Unleashed Air Racing
P.S. Qualified at 268.781 mph this year!