A long strand of 550 cord, or other tiedown cordage would work best. Heavy twine would work, too. You just anchor it at one end, and wrap around the load, weave through the rack or bike frame, wrap back across the load, etc etc. Make sure to criss cross, and pull it uniformly tight. Tie off with a good knot, you're done! You could probably stuff enough in your seat tube, and forget about it until you need it.