With the lid slightly closed and leaning towards you, place your fingers on the outside corners of the lid and place your thumbs towards the center underside of the lid (on the aluminum near the camera). Flex it very slightly and carefully. The gaps in the lid when closed will be lessened. If the gap you have is bigger on one side than the other, exert slightly more flex on that particular side.
This isn't my idea. When I took back a faulty Macbook Pro to the Apple Store (for a different problem), I mentioned the additional annoyance of the poorly closed lid. The Genuis said "watch" and he did what I described above, and it was fixed. He said they do it all the time.
If you think it sounds wrong to give it that little flex, think again. When the lid closes, the central catch forces the center of the lid down and the upper edges can bow up. So it's being flexed all the time it's closed anyway. All you'd be doing is correcting it. Aluminum is soft even the pressure on the lid from the latch can cause it to bow from the center. Correcting it is easy, unless you're heavy-handed and need to keep away from anything requiring a delicate touch..
