Do you realy need the stiff plastic? My commuter bag (a cross between a bike trunk and a courier bag) is just silnylon, a zipper and two velcro tabs on either side. Its realy long and wide and works just fine. Roll top closures are not difficult.
Why do you want a roll top closure? Are you trying to make it water resistant/proof? If you are just trying to make a neat top that does not require a lid, then there is an easier ways of doing it, but they are not water resistant.
Some pics of a very flimsy but effective roll top...http://i2.photobucket.com/albums/y23/tbdesigns/sewing/comutetrunk8_8_1.jpghttp://i2.photobucket.com/albums/y23/tbdesigns/sewing/comutetrunk7_7_1.jpg