The sides were joined together front and back at the correct width, but of course this didn't produce the right width in the middle. Even some bracing across the middle wouldn't bend the plywood enough to reach the correct width.
So I began building up the sides to the correct profile with the application of some shaped shims.
I then wrapped the frame with construction card to form templates for the metal sheet.
I've really no idea if this is going to work, so I'm just going to press on and see.
The current plan is to fold the metal around the horizontal parts and glue the verticals on with a glue that will glue metal. If that doesn't work I'll screw the sides on and cover the screws with Bondo (P38?).