What we found worked for our extremely old cat when the constant yowling started was a kennel, as bad as that seems. During the day he was noisy but ok if someone was around him. At night he just yowled and cried. We got a medium sized dog kennel so he would have room to move, put some cushions in it, put him in there and covered it with a blanket. He would go right to sleep. We would leave him in it for only a max of 4-5 hours though, we had housemates on different schedules so this worked for us. We did eventually decide to put him to sleep though, he was just too old. This worked for a few months though!