All the ply builds i've seen have reinforcing fibreglass strips where the sheets of ply meet. On the front of your proposed build your fibreglass strips would have to be attached to ply and viewing panel!! and not the traditional ply to ply way. That seems odd to me. As above, you need to go bigger and sit that panel in a jigsawed out piece of ply where the hole in the ply is smaller than your viewing window so you have the overlap.