I've cut a section from a pop bottle ( chop the mid section to the width of your out flow of your filter) cut that mid section, now a cylinder, so if you were to "un-roll" it you would have a rectangle ( I know this may sound confusing) tape one end to the top of your filter and let the other end go down and wrap around the out flow part.
When the water comes out it will hit the plastic and lose momentum then go to either side. I did this for a fry tank once because they were having a hard time swimming.