Use two thin pieces, each 1/8 inch (?) thick. You can probably eyeball the thickness. Cut one to fit the inner side, cut the other to fit the outer side. They will differ by about 1/8 to 1/4 inch per side depending on your camera. It is this "lip" or "staristep" that keeps the light from leaking around the board. Then put them together with wood glue and some clips until the glue dries. Or, you can use black pieces of plastic if you want. To find the center for the lens, draw two lined corner to corner. Where they cross is the center. Use old, seasoned wood. New wood will warp. One reminder.... it is better to cut off too little each time. You can always take more off. You can't put it back on.