No doubt you have a plan Will, but I if I had to re-make that panel, me being stubborn, I would spend some time shaping a bit of seasoned wood, say about 9" x 3" to fit pretty tightly in side it, then I would clamp or screw the new metal to it and bend it round, making V cuts where necessary with a thin 4" cut off disc, a few small holes for screws into the wood to hold each bit in place whilst tack welding. (Make it oversize and trim to fit when all shaping is done) Finish welding, a bit of adjustment my be needed after welding to get it snug on the wood, then fill and prime, it works for me, and cheap.