14 gauge sheet steel from Alaska Steel, bent on my brake and butted in. A brake with a 1/8" bend radius is great for aircraft aluminum, but usually not as good for automotive work. In this case, however, it produced a near perfect match for the factory bend. In hindsight, I probably took out more metal than I really needed to. But it turned out well.

While you're in there you may find something like this. It is blocks of mineral wool installed at the factory prior to welding the frame together. Purpose is for sound deadening. It reduces pinging and ringing sounds when the frame may be peppered by road rocks or otherwise subject to high frequency vibration. I don't think the stuff promotes rust. The MGA frame rusts from the outside and is typically very clean inside, until the outside rust breaks through and allows water inside.
(Photo compliments of Mark Vine in Essex, England, UK).