You might as well go ahead and buy this package because you will likely end up adding the extra parts later. It holds in the smell better than just having an open top. My 2 kittens took to it immediately. I didn't even have to get them used to the cleaning cycle. They just stand there and watch it while it's cleaning. The only bad thing is that my kitties will jump right back in it after it's clean and just scratch around, but this activates the machine to clean again. I haven't figured out which setting I want to use yet. I like cat activation to help avoid the smells, but since they like to jump in it right after a cleaning just to play around, I may have to put it on a different setting. Regardless, this is a fantastic product!