Material has been discovered. It is HDPE. High Density Polyethelene or something like that. Basically, cutting board. Really big cutting board.
Paul P used this to plate the bottom of his rally ute that won class in ARC too. Ran it for years, he said there were big gouges in it but nothing ever went through. He thinks he was using 4.5mm, but they come anywhere from 3mm to 50mm thick. Cheap stuff too, generally about $50 for a 2x1m sheet, the cheapest quote I got was like $90 for an 8x4m sheet, most expensive quote I got was $112 for a 2x1m sheet, that's for 6mm stuff. If I feel hardworking and motivated enough and end up doing this, I'll probably use 4.5mm stuff too.
It's light, it's reasonably flexible, but tough enough to deflect rocks etc (of course if you come landing hard on it you'll be in trouble, for stuff like that use a proper sump guard like what Rob has...), it's cheap, flame retardant (it basically melts, doesn't really burn at all), perfect stuff.