I have a clumsey but sure fire way to find the center if your circular firewall. I would use my mechanical drawing dividers or compass - or any drawing compass for that matter. Place the point of the compass where you think the center is and move the drawing point around (don't make a mark, substitute a piece of sharpened dowel if you want) move the point until you can "draw" a circle within the confines of your cowl. There is the center. The diagonal line piece is OK if you know the widest points of the circle. Like I said, my method is clumsey but effective - and sure fire.

Bill