My first internal frame was bought from a small manufacturer in Chico, Ca called Caribou Mountaineering. It had a huge single bag and a web system to lash on tools on the outside. The supports were 2 strips of African Basswood, roughly 1 1/2"x 1/4" which would bend but not break. I think it's the only pack I've seen which used wood supports. The strips were fitted into 2 4" sleeves top and bottom and you could remove the strips if you needed to. I gave it to a friend and it still sees a trip a year or so, after nearly 40 years. Is your plywood going on the inside of your bag? Most grades of plywood over time, with stress and drying, will de-laminate unless its marine grade. Don't mind me, I'm just a regular old carpenter. That laser cutting is impressive.