I used a couple pieces of poster board to make a pattern for what I had in mind for the new firewall top flange.

I made this elaborate jig for a welder to use. The idea was that a welder could cleco the flat crescent-shaped piece to the board, wedge the flange into the top of the arc, use the smaller shims to hold everything tight and then weld the thing into shape.

Long story short, but I was hugely disappointed by the welder's work. I have since learned that no matter how macho a welder pretends to be, it's somewhere between difficult to impossible to weld 2024T3 aluminum.

This was the original plan. I'd have one welded up flange facing forward for the fiberglass cowling to attach to and one facing aft for the sheet metal covering the gas tank to attach to. I could trim the flanges as necessary... and I'd sandwich the original firewall between the two. Once both pieces were attached, the top of the firewall would be stabilized fore and aft. All should be right with the world...

Fast forward a year or more and I'm ready to put my plan into action. I decided I might as well remove a little excess material so I drew a curvy pattern on the flange. I knew I'd need to bend the flange slightly beyond 90 degrees to account for the angle at which the cowling meets the firewall. [Approximately 9 degrees beyond a perfect 90 degree angle, for those of you keeping track.]

I used a Unibit to define the radius at the bottom of my swoopy lines...

Since the piece was too curvy and big to cut with my bandsaw, I ended up using a die grinder with a cut-off wheel to cut the angles. I then used a small drum sander in my drill press to sand the edges reasonably smooth before I then used a Scotch Brite drum mounted on a 1/4" arbor and chucked in my die grinder for the final finish. Yep. It was a thing of beauty... Kind of like the Hindenberg!