I use the ziplock sandwich bags to do scud backs. If you look colsely at the bags you will see that they appear to have a grain or knap in the plastic. You'll want to cut your strips so that you follow the grain, not cut against the grain. If you cut the strips against the grain, when you put a little tension on the bag when tying it down, it will stretch. Cutting with the grain doesn't cause any stretch. Usually the grain runs parallel to the zip lock.