Synthetics don't split or crack like wood can. So if you are just learning to peen, it is easier to start with G10, micarta or acrylic. Also, I find that mechanical screw type fasteners are easier to install. My advise would be to make it as easy as possible for your first attempts. A successful first try will give you more motivation to keep going and keep learning. Give yourself as much chance of success at first and then add variables and more challenging materials and techniques as you build your confidence and experience.