Actually, the construction technique used for shields wasn't too far off from plywood. They were usually constructed from 5-6 thin layers of lime wood laid out in alternating rows (the first layer is horizontal, then vertical, then horizontal again, etc.). Lime was preferred because it resists splitting, but is also very light. Combine that with a facing and backing of linen or leather, and a leather rim, and you now have a shield that even if the planks ARE broken will still be usable, and is also quite lightweight.