Ok, so that was a giant PITA. Brought the metal piece to a shop. They were convinced that it was just 16g mild steel. The piece that I had cut seemed a bit thinner. We'll see how it holds up. Unfortunately, removing that manifold again required me removing the top, front, and back of the stove. Took forever to reassemble because of cleaning off the old cement. I ended up grinding the edges of the air chamber before reassembly so that it can be removed in the future if needed. Worst case, I'll need to remove the top and the rear baffle. Bending the sheet metal was a trick as well. The sheetmetal brake that I have couldn't handle making the middle bends in opposite directions, nor could it handle the edge bends. So I ended up using a vise and BFH, which is not a precision method. It came out pretty good in the end though, and if a secondary burn is possible on this stove, it's going to happen now.
Old sheet metal. It had warped and separated from the edges of the air chamber, essentially not routing any secondary air.
I ground down the front edge and the relief in the middle (where my finger is) so that it could clear the mounting shelf. It looks like it was designed to be removed without all the drama, but I had to disassemble the last time I did this as well. Not doing that again.
It ended up fitting the cast iron pretty well. Not going to win any beauty contests, but I think it will work fine. The fit from the factory was pretty sloppy; when I bought the whole chamber 5 years ago I ended up re-cementing the seams.
Finished product. My hole drilling also wasn't pretty. I was starting to run out of patience...
Most important thing is that the air chamber is leak free and seals with the rear baffle.
For $30...I dunno, a lot of time. Hopefully won't be a next time, but I'll be able to do much quicker if I do.
Now just need to sweep the chimney, and we should be ready to roll!