Depends on your design, really. A badly designed chassis won't stand up to anything, be it wood, HDPE or hardox.
HardWired II uses many angles so it can deflect spinners rather than taking the full force of the attack - just watch my fight vs NST, they're very effective.

A bolt together chassis can definitely work, in fact I'm fairly sure there's quite a few machines out there that literally are just bolted HDPE, I know Hatchet is and I think Loki is too, but I could be wrong. As for thickness, if you go around 20mm you'll be fine. You could go less but it's just dependant on your design - again, the design is key, it's not so much a case of just bolting it together at the sides - unless I've got the wrong end of the stick here!

Either way, I'd look at the construction of a few machines in the build diaries section, and go from there!