I think I'm going to remove the straps and use Zips to the buckles on the bottom, which seems to be the common thought but I don't want to drill a hole through the side to attach the frame to the top half.
I'm toying with the idea of using the straps to just wrap around the top of the frame and then using a Zip to lock it so it doesn't slip.
I should mention that I intend to buy some wood next month and make a proper back as well as install pack electronics.
So the the solution doesn't have to be perfect, just good enough to get through a few hours on Halloween.
Larger picture and Image of the lower section in the link. https://imgur.com/a/G7icn
I tried to size the frame to the pack so it isn't sticking out the sides, not sure how comfortable it will be in the end but the pack is light so it shouldn't be a problem as long as it is wide enough for the straps.
I've worked the bottom so that I can attach a piece of PVC with foam with cord, the red is just a stand in, I've got black around here somewhere, just needed to make sure it would work.
This way I can replace it with a proper pad in the future if I want and I can adjust the depth of the foam by cutting a slot into the PVC.
I did not add a bend to the top as I plan to attach it via the straps somehow, instead I've added a 2nd layer of foam which should provide a similar effect.