This first photo shows a ply blank I made 5mm smaller than the aircraft's actual firewall. The intention is to bend SS tabs forward as a means of attaching the engine cowl. I have placed the engine mount in place as I am using it as a reference for the final layout.
This one shows the layout drawn on the SS. I later discarded every second tab as a weight saving exercise.
Measure twice, cut once.
Cut to shape...
And bend the ears forward. Before I did this I used the router to round the inner edge of my template so I didn't set up a stress in the SS while bending it.
This one shows one of the the augmenter openings and the firewall extending below the original belly line. I intend to carry a 2" deeper false floor to enclose the augmenters and it produces a pleasing line from the bottom of the cowl.
This last one just shows the Throttle and mixture cables being mocked in. In the previous you can see the gascolator temporarily placed. I left the ply in place so I can mock in all other firewall attachments by screwing into the wood prior to actually drilling the SS behind it. Measure twice, cut once....