Step 2: building up (part 1)
At this point, I used a metal mesh body patch (bondo brand) to fill the hole, replace the missing metal, and give the bondo a place to adhere to.
Using regular scissors, I cut the mesh to approximately the right shape, and then began fitting it into the hole between the metal and the foam. Along the bottom, I had to loosen the foam from the metal a bit using a screwdriver. Since the area is curved, I had to shape the patch as I worked, and getting the last (upper left) corner in took some work.
The body patch is self adhesive, and has a removable backing. You can see in the photo where it crinkled up while I was maneuvering the patch into place. According to the instructions, I should have taken the patch back out, peeled off the backing, and then re-inserted it. But once I got it in and shaped, it wasn't coming back out, and I decided that I don't care if there's a plasticy sticker bit in there.