Having built a couple (You've seen my other posts!), half-inch plywood is plenty thick. With the leather coverings and the padding in the back,the finished targe will be about an inch thick. They're meant to deflect sword blows and will stop an arrow. Musket balls, however, zip right through them...!

Yes, the ancient method was, in effect, plywood - two layers of planking glued and pegged togther with the grains running at right angles....