Help needed: backpack for heavy loads

I want to sew a rolltop backpack for multiday packrafting tours with about 70l volume. Usually I carry about 20-25kg of gear on my trips. Thinking about my new project I ask myself how I can design the backpack for this weights. Do you have any tips for the design of the frame? What are the Most important design features for heavy loads?

I want to use XPAC21 for the backpack as main material.