I hate to do this too you but are you putting a back pad at the illac crest of your back ( the bit that curves towards your ass) ? That may interfere with the hipbelt snug fit ? If you think about it when you walk the rucksack is going forward and thrusting down your lead leg by driving into your back. If you get good contact you get good load transfer. Alot of the best light bags ( exped lightening ) do not skimp on the padding only on the rest of the bag.
I like the plywood frame shame its so heavy, a couple of aluminium stakes only weigh 4 oz. What alot of companies do is have a stiff foam back, sometimes reinforced with thin steel rods.
What weight are you looking to carry ?