Swiebertje is quite correct. Displacement is related to mass and the volume is irrelevant. If you want to prove it float something in your bath and add first something heavy and small. Then try something the same weight but bulky. You will find it sinks (displaces) the same amount.
So if you fill your container with a sleeping bag it will make very little difference to the buoyancy.
If you think about it this is how a foam filled buoyancy compartment works
Hope this clears up your misconceptions. Robert