I get the drift that you want to modify your bag as opposed to buying a ported bag.I have a ported bag.So I'll tell you how they did mine.You'll need industrial strenth "sticky".
You either use a piece of flat sheet rubber with,or without one side being sticky(if not sticky you'll need some good glue).
But basicly you cut two pieces the same(one for inside,and one for outside)'then stick them on facing each other(glue side toward cloth).then after it dries cut the rubber like a pizza ( 8 pieces ) without cutting all the way to the edge.Then you can poke the cord(with jack-plug)through the port.
If you can find a fabric store(big),they usually have some heavy duty hook tape(wide roll),it'll come in two parts one will have the hooks and the other will have the foozy cloth.For this project just use the foozy part only(save the rest of roll,you'll love this stuff).
The other thing is,I have used bags without ports,and really haven't had any leak problems running the wire though the zipper.I usually keep it closed with the two zippers coming together at the right or left rear corner,not middle rear.Water doesn't seem to collect to the side rear.