I use an old Kelty Trekker external frame with Sea to Summit eVac dry sacks instead of the original heavier Kelty bag. I made one vertical and two horizontal ½” wide straps to lash the dry sack to the frame. The long strap that I use in the vertical position doubles as my camp tree strap for hanging gear for easy access.
For multi day winter trips, I use a 65L dry sack(pictured). For summer I use a 35 liter dry sack. The system works great. The frame, dry sack, straps and water bladder sack runs just under 3 pounds.
I love that my pack always fits no matter what I have strapped to it.