I use 5/8 and 1/4 inch plywood. I have made them from baltic birch, maple ply, and teak ply. Many make them with marine ply. I foil them until they “look” about right. Send me a message with your address and I’ll send you some templates if you’d like. Mike
They’re about 9" long and 5" deep. The wood layers help guide the foil. Look at pictures of other fins to help you.
The glass center can be achieved easier with a Lexan core. Don’t know the ideal thickness. Thickest point is just back of the leading edge. On surfermag’s design forum someone posted the templates a while back.