A piece of black starboard. It's impervious to the elements, including UV, has a nice crinkle finish, and can be easily worked like wood. I've made some nice trim pieces for my boat using a bandsaw and trim router, but it can be done with even less in the way of tools. Being like wood, it's easy to work with, and never needs paint touch-up. Thickness available ranges from 1/4 inch to 1 inch. I find 1/2 inch to be the most common "right thickness". You can get supply pieces as small as 12 x 12 inches from various websites, even Amazon.