When you see which area Kitty is using, you can provide this depth in all his boxes. Some cats just seem to have peculiar preferences for toilet areas and seek out smooth or soft surfaces. Try offering the cat an empty litterbox or one lined only with newspaper. Never punish the cat for eliminating outside of its litterbox. Housesoiling happens when the litterbox, its contents, or its location is offensive to the cat or when the cat is stressed by the environment.
Punishment only increases the cat's stress.
If aversion to the litterbox can be ruled out, consider that the problem could be anxiety-related. Any intrusion on the cat's territory, whether human, animal, or even a new piece of furniture, can cause a cat to feel threatened, insecure, and stressed. This results in his need to remind himself and the world of his territory. Territorial marking is usually accomplished by spraying urine on vertical surfaces, or less frequently, by squatting and urinating or defecating on horizontal surfaces. Try to relieve or eliminate the source of the cat's anxiety.