Balsa is hardly light proof until you get way past 1/4 inch.
Whatever wood you choose should be tested. Go to a dark room with a very bright flashlight (torch) and wait for your eyes to adjust. Then put the light behind the wood and shine it through, try to seal the light so that none can escape around the sides. If you see any sort of dim glow, the wood is not a good choice.
I once though 1/16 thick aircraft grade plywood might be a good choice, I found after gluing two sheets together (now 6 ply and 1/8 thick) that it still let far too much light through. Took about 4 heavy coats of black paint on the inside and outside to finally block everything that I could see and not fog film when left in the sun for several hours. Lessen learned there.