We decided we wanted to keep the dog we are getting out of the room with the litter box. I suggested a gate, but my wife was worried about forcing her 15 year old cat to jump to get into the room. She found the kitty pass xl on Amazon.
They advertise it as being big enough for a 30lbs cat. Our 16lbs cat fills it for the most part; so I doubt that claim. All the cats have used it though so I’m calling it a success.