This allows the cat an escape route to the right and left from an ambushing cat. Most cats can be very nervous in an enclosed space in new surroundings. The lid can gradually be put on. These can aid in keeping the box cleaner. Most of all, make sure you wash the pans out at least weekly with a mild detergent.
Strong fragrances can put off a cat from using the litterbox.
Hopefully with these tips, you'll have perfect litterbox attendance from all your cats. If they use the box perfectly now, leave it alone.