You may also try and adjust your trunk lock catch a little tighter.

If your trunk closes "too easy" when you shut it, it may not be sealing proper.

When I first bought my car I could close the trund and push down on the spoiler and feel the trunk compress more so I adjusted the latch so the deck lid had no play.

I never got a chance to see if it would have leaked so it may not help you, But it probably wouldnt hurt to try.