+1 for Sir Newton's comments.
I've made 2 packs with similar drop-in style plastic framesheets with alumnium tube stays. I think they work fantastically well and are very easy to design and construct (a rectangular sleeve on the inside back panel, plus some kind of flap over the top). Sorry I don't have any pictures handy but what I have done is very similar to Newton's setup, except I used tiny zip ties instead of a webbing sleeve to attach the stay (or stays) to the framesheet.
Check out cilogear packs (like the midsize worksacks), and look at the way they use a framesheet and stay like Newton's, plus a folded foam pad for cushioning (and multiple use as a sleeping pad), plus the "ninja pocket" as both a closure for the top of the framesheet pocket and a way to add a little internal organization. It's a neat little design, works well, easy to construct, plus you can change out the plastic sheet, stays and foam as needed until you find the combination that meets your goals for stiffness and light weight.
No need for turmoil! This will all work out just fine! :)