I would think it would be more stable if it was left upright, or upside-down, and the hole cut in the bin rather than the lid. Doesn’t it rock when the kitties move around?
The bin is on the side, which is flat. No, it doesn’t roll around when they are inside or entering or exiting. The hole in the lid is about the size of a cat door and is at one end, making a nice little cat cave where they can curl up together and get out of the direct wind.