I wonder...

Could you take an old prop spinner.. Big enough so edges of the spinner touch the inner circle of the cowl. Drill a hole in the tip of the spinner (if it doesn't already have one). Then put the spinner into the cowl (pointy end towards the firewall) Make shere the base of the spinner if parallel with the face of the cowl...

Once the spinner hits the down, the point will automatically be center.

Viola! Mark it, or what ever.. it should be REALLY close.