Why not make the Kammback out of the rear window? Just make some spacers to git the right angle and then you could have it made of a more durable material so you could lock it.
2mm should be fine if you have good framing for it. Thin guages are good for just bending onto a curved shape, while thick guages, 4mm and up, are more suitable for holding a shape without a frame after heat-forming.