I went to the army surplus store and got two military supply carry-arounds. They already have clips on them, so I just clip them to my back rack. They were 4 dollars each.
Swiss Bread Bags (another military bag) work very well, also. Bread bags (or ration bags) are usually water resistant. You should look on ebay for some of thems.
Military bags usually have all sorts of straps all over them so it shouldn't be too hard to mount them to your moped.