DC, That is the spot!!! I think????? I followed that angled piece of balsa stick between F3 & F4. 1/8 is not needed. 1/64 is plenty strong. You just need to tie all the bits and pieces together. Maybe even remove some of it in the open bays areas as you won't have to contend with vibration like a glow motor produces. You could use a long skinny balloon to hold the ply in place while the glue dries. I'm thinking out loud on the balloon thing
come to think about it, I added some 1/64 to the rear of the wing saddle as it looked "weak" to me. Use your own judgement there.
Look good so far!!