at this thickness it would be something like aluminum, not steel. Good as a frame material, while softness make it not that good for point penetration resistance (it would be somewhere like 1mm steel equivalent, so seems i was wrong above). 5mm Al cover for 10ftx6ft battery would weight 80kg - doesn't look like there is much room for increasing.

So it doesn't sound like they did a screwy job - that's good to hear (as i said i'm a fanboy of Tesla and think nothing can stop them but their own hubris - when/if it appears), yet it also may mean that there is no easy, "low hanging fruit", way out.