I actually have a dripper I didn't use a plastic bag. It's a bottle with a hose coming out the bottom, it has a nozzle that you can adjust the amount of flow you want from a few drops a minute to a steady stream of drops. It works much better then a plastic bag IMO. You can easily pour water back into the bottle with out touching anything and have the drops continue to fall in the same exact spot.
Here is another shot with the nozzle opened up a lot, you can see the difference.
