Did you do the exhaust hole yet?
I did mine yesterday and it was perfect. (look at my photo in my build)
I took a quarter inch wide scrap of aluminum strip and taped it to the bottom of the stainless firewall where it's flat under the plane. The strip pointed directly at the center of the pipe and touched the back of the pipe.
Then I took the muffler off and slipped the cowl on. The flexible pointer laid right on the inside bottom of the cowl. I marked the center and tip which gives you the back edge of the exhaust and the center left and right.
I punched a hole the size of the pipe and then enlarged it with a dremel wheel sander by hand. The curve of the cowl forces you to make somewhat of an oval shape if you maintain perfect spacing around the pipe. I left about 3/8 inch all around.
Back to the boot cowl. I was curious about painting the screws down the post next to the door.