Use the same straps that your BMW Motorcycle was shipped with. These are the actual lashing straps that BMW Motorrad uses to tie down the bikes in the crates at the factory.
If they are good enough to hold down your bike for the trip across the Atlantic, then they should be good enough to tie down a sleeping bag.
Over four feet long.
BMW Motorrad Factory Lashing/Tiedown Straps - 4 Straps USED