Wood , mother natures own carbonfiber.
Depending on kind of wood and treatment, not to be underestimated. If 18th century ships of the line could stand up with oak and teak armor against heavy cannons, why would a robot do less?
Warrior, the first British Ironclad, used 18 of teak to back up the 4.5 of iron armor. (incidentialy, she's at berth in Portsmouth...)


The Series 7 veterans here have seen how wooden armor destroyed an Etec.

The Dutch Robot Twister, using a heavy triangular sledgehammer directly mounted on the Etec against Brutus Maximus. The wooden armor of Brutus maximus stood up well, especialy compared to the more common ali and/or polycarb armored machines.

Just think about the difference in cost... Plywood vs Etek....