Given the need for clamping panels together, how did you manage to clamp that steel roof to replace the fabric roof on the Model A? I suppose that weights would do the trick as long as the panel was horizontal, but if it were vertical, it would be a challenge. Magnets probably wouldn't do the job, as the attraction between the two sheets of steel would be minimal, but maybe a magnet on each side would work. Just curious.
I would think one way would be some really long clamps. Maybe two pieces of wood strapped together, one over the roof, the other through the door openings, then use wedges to clamp things. The are multiple ways to apply pressure to something.
