Don't worry about it. Like Kephren says, fermentation will produce pressure inside the bucket, that will keep bugs from "dropping in." Also, if your yeast is producing krausen or foam and crud on the surface of the beer, it is healthy, happy and crowding out most bacteria that might want to infect your beer.
After a year of sitting idle, Andre's All-Grain Brewery has reopened.
Batch #59-Kolsch (Secondary)
Batch #60-Blueberry Ale (Primary)