Completely agree 4-6mm HDPE on its own isn't anywhere near enough. But is does help the Hardox by absorbing the impacts. What you've got to remember is that the frame is only about 10mm wide, so the HDPE is quite crucial. I think that the HDPE gives a lot more strength than it looks like.

But I'd say without a frame you need at least HDPE 10mm thick.

I'm also inclined to agree about the excellent design and superb driving comments