I actually just did this last weekend.
You can go to (gulp) Home Depot and get wome weather stripping - commonly used on doors and windows - though it may also have a picture of a sailboat
hatch on the front. I outlined the stationary part of the opening so that when the lid closed, it sealed it completely. THe is also nice because it will reduce/elminiate the rattle from the lid from vibrations (from air conditioners, for example).
The tape is very inexpensive and it is a VERY easy job to do. I finished mine in less than ten minutes with a roll and a pair of scissors.
I hope that helps.
- CD
You might also try the weather stipping that is a rubber gasket. I did not bother since I have done it this way before.