I began by taking an old pack that I don't use anymore and cut off the sides and the lid to get this (origanal pack size was 40litre)
The pack body is replaced by a drybag, in this case an 80litre pack liner for easy packing, as shown here
Note, I don't actually carry that much stuff, just using a liner that size to make packing easier.
Weight is approx 1.8kg down from approx 2.5kg so not exactly as light as some of your packs, and I've converted the lid to a detachable front pocket, which can either be used for things that easy access is required to or for crampons. I've left the bladder pouch in place but will probably replace that with some vecro, Not sure if I can reduce weight any more than this though without making comprimises. I don't want to remove the bottom compression straps for obvious reasons, nor do I want to remove the mesh side pockets or bottom compartment fabric as I'll be using them for bottles and wet gear respectively.