It's not sheet metal. It's plate on the Model X. I'm not sure there's a place to measure thickness, but I'll try to remember to look today as it's a good datapoint for people to have.
There are crosscar rails inside the box that are welded to the plate as well, which are placed about 13 inches apart (module width roughly) so thickness gives you puncture resistance, but that and those rails prevent bending.
Alloy is another factor but I'd have no idea which it would be.
All that was not good enough and a recall retrofitted a titanium deflector plate in front of the pack after road debris encounter incidents.