Inside or outside of the top?
Inside, use a mix of 3 parts water to 1 part bleach and allow to dwell (10 inutes)... Wipe clean and your're done. (Need good ventillation and don't get the solution in your eyes).
New air freshner or some Frabreeze to neutralize the bleach...
Outside - the sun is the cheapest sterilizer going... simply bake in the sun, rotate every 20 minutes, wash down.