The plan was to get a single large sheet of sheet metal, about 2ft x 3ft. But I couldn't find anything like that at Home Depot. What we found were 1ft x 2ft sheets, so we got three of them and some plywood to serve as a backing (more on why that did not work out quite as well as we hoped, later).
The first step was to router the back of the trim in order to make a groove (slot? channel? lip?) the sheet metal would sit in. Here I've clamped it to the table in the back yard (backwards).
The routing finished.
Next I cut some 45 degree angles on the trim pieces and began dry fitting the metal and the trim on the plywood. We got a 4x4 sheet of plywood so we had to figure out the final size.
Of course, you get what you pay for when you shop at a giant store like that, and our plywood was slightly bowed. Here we've glued the metal down with Liquid Nails and we're using some water bottles to act as weights to make sure the bond was tight.