Stone Glacier has Load Cell Bag for that purpose. 2500cuin, 4.5oz.Currently running a 30 litre OverBoard Dry Flat Bag between my bag and frame on the K3 3200, adds another 1800 CI, when needed. Also running a 6400 for 10-day trips. Hoping Exo comes out with a flat dry bag to run between the bag and frame. Thinking I can almost run a 5000 for everything, with a dry bag on the load shelf, when needed.
That works great. I use a Zpacks small bag liner for the same reason. Check them out if you want to shave some weight. I use it with my 1800 for 2-3 day trips