Frameless packs aren't designed to carry heavy loads. Figuring out your base weight and your average weight of consumables will get you started. The Ossa 48L is a framed pack can be stripped of its frame and used frameless also.
If you use an inflatable mat you can fold it to the dimensions of the back panel or you can use the back panel foam option. Using either of these is made easier by adding an internal pad sleeve to hold things in place while loading your pack.