Cowling 101
Jeff's right. Before I starting chopping my cowl, I spoke to Jeff for about an hour out of fear I'd have to tell the wife I just destroyed a cowl and needed the checkbook again.
But we gradually trimmed the nostrils enough to allow the center section to come together. Then we trimmed the sides of the center a little to bring even more of it together. My build partner Chet says "we're sneaking up on it" and that's about right. In other words, approach it gradually. Using Jeff's other good idea, Chet made a wooden backing plate for the spinner so there was no flex giving inaccurate measurements.
Make the bottom half fit first cause you can swivel the top to mate to the bottom. Remember that, unlike alum, you can easily add material back to the cowl so keep your WEST kit handy.
We trimmed the cowl using an el cheapo Harbor Freight 4" angle grinder. Because we are building a 9a, we cut the gear leg opening with a 1 & 1/8" slot at first (versus the 1 & 3/8" required opening) thereby leaving material for a second cut when we found the true center. You'll find that the flat side of the angle grinder blade will sand nicely.
Cutting an underside gear leg slot was a good idea too.
Last, Chet made me a 4' long board for sanding the bottom edge of the top cowl. He glued 80 grit paper to it. That edge establishes the paint line and it better be straight or the plane will look like ?*&^%$.
Our cowl is now mounted using a huge bungee cord. It's a good thing I didn't screw this up cause the wife hid the checkbook.